Remove useless GET_TRANSACTION_POOL_BACKLOG endpoint

Not only is this endpoint not used anywhere, but it returns (invalid)
cancerous json output when invoked in current stable oxend:

    {
      "jsonrpc": "2.0",
      "id": "0",
      "result": {
        "backlog": "l\u0007\u0000\u0000\u0000\u0000\u0000\u0000�\u001f�\u0000\u0000\u0000\u0000\u0000Y�������",
        "status": "OK",
        "untrusted": false
      }
    }

where that cancer in the backlog field is the raw bytes copied out from
under a struct with 3 uint64_t's.  (It's also invalid json because those
unprintable characters are \xff, which isn't valid UTF-8 encoding, and
thus isn't valid JSON).

Just nuke the whole thing since nothing uses it (probably because it is
such cancer).

src/cryptonote_core/tx_pool.cpp

@@ -1163,18 +1163,6 @@ namespace cryptonote
     }, false, include_unrelayed_txes);
   }
   //------------------------------------------------------------------
-  void tx_memory_pool::get_transaction_backlog(std::vector<rpc::tx_backlog_entry>& backlog, bool include_unrelayed_txes) const
-  {
-    auto locks = tools::unique_locks(m_transactions_lock, m_blockchain);
-
-    const uint64_t now = time(NULL);
-    backlog.reserve(m_blockchain.get_txpool_tx_count(include_unrelayed_txes));
-    m_blockchain.for_all_txpool_txes([&backlog, now](const crypto::hash &txid, const txpool_tx_meta_t &meta, const cryptonote::blobdata *bd){
-      backlog.push_back({meta.weight, meta.fee, meta.receive_time - now});
-      return true;
-    }, false, include_unrelayed_txes);
-  }
-  //------------------------------------------------------------------
   tx_memory_pool::tx_stats tx_memory_pool::get_transaction_stats(bool include_unrelayed_txes) const
   {
     auto locks = tools::unique_locks(m_transactions_lock, m_blockchain);

src/cryptonote_core/tx_pool.h

@@ -43,7 +43,6 @@
 #include "cryptonote_basic/verification_context.h"
 #include "blockchain_db/blockchain_db.h"
 #include "crypto/hash.h"
-#include "rpc/core_rpc_server_commands_defs.h"
 #include "tx_blink.h"
 #include "oxen_economy.h"
 
@@ -410,15 +409,6 @@ namespace cryptonote
      */
     void get_transaction_hashes(std::vector<crypto::hash>& txs, bool include_unrelayed_txes = true, bool include_only_blinked = false) const;
 
-    /**
-     * @brief get (weight, fee, receive time) for all transaction in the pool
-     *
-     * @param txs return-by-reference that data
-     * @param include_unrelayed_txes include unrelayed txes in the result
-     *
-     */
-    void get_transaction_backlog(std::vector<rpc::tx_backlog_entry>& backlog, bool include_unrelayed_txes = true) const;
-
     /// Return type of get_transaction_stats()
     struct tx_stats
     {

src/rpc/core_rpc_server.cpp

@@ -2641,19 +2641,6 @@ namespace cryptonote::rpc {
 
     sync.response["status"] = STATUS_OK;
   }
-  //------------------------------------------------------------------------------------------------------------------------------
-  void core_rpc_server::invoke(GET_TRANSACTION_POOL_BACKLOG& get_transaction_pool_backlog, rpc_context context)
-  {
-    PERF_TIMER(on_get_txpool_backlog);
-    //TODO handle bootstrap daemon
-    //if (use_bootstrap_daemon_if_necessary<GET_TRANSACTION_POOL_BACKLOG>(req, res))
-      //return res;
-
-    std::vector<rpc::tx_backlog_entry> backlog;
-    m_core.get_pool().get_transaction_backlog(backlog);
-    get_transaction_pool_backlog.response["backlog"] = json::parse(backlog.begin(), backlog.end());
-    get_transaction_pool_backlog.response["status"] = STATUS_OK;
-  }
 
   namespace {
     output_distribution_data process_distribution(

src/rpc/core_rpc_server.h

@@ -217,7 +217,6 @@ namespace cryptonote::rpc {
     void invoke(GETBLOCKCOUNT& getblockcount, rpc_context context);
     void invoke(MINING_STATUS& mining_status, rpc_context context);
     void invoke(GET_TRANSACTION_POOL_HASHES& get_transaction_pool_hashes, rpc_context context);
-    void invoke(GET_TRANSACTION_POOL_BACKLOG& get_transaction_pool_backlog, rpc_context context);
     void invoke(GET_TRANSACTION_POOL_STATS& get_transaction_pool_stats, rpc_context context);
     void invoke(GET_TRANSACTIONS& req, rpc_context context);
     void invoke(GET_CONNECTIONS& get_connections, rpc_context context);

src/rpc/core_rpc_server_commands_defs.h

@@ -1278,29 +1278,6 @@ namespace cryptonote::rpc {
     static constexpr auto names() { return NAMES("get_transaction_pool_hashes"); }
   };
 
-  OXEN_RPC_DOC_INTROSPECT
-  struct tx_backlog_entry
-  {
-    uint64_t weight;       //
-    uint64_t fee;          // Fee in Loki measured in atomic units.
-    uint64_t time_in_pool;
-  };
-
-  //-----------------------------------------------
-  /// Get all transaction pool backlog.
-  ///
-  /// Inputs: none
-  ///
-  /// Output values available from a public RPC endpoint:
-  ///
-  /// - \p status General RPC status string. `"OK"` means everything looks good.
-  /// - \p backlog Array of structures tx_backlog_entry (in binary form):
-  /// - \p untrusted States if the result is obtained using the bootstrap mode, and is therefore not trusted (`true`), or when the daemon is fully synced (`false`).
-  struct GET_TRANSACTION_POOL_BACKLOG : PUBLIC, NO_ARGS
-  {
-    static constexpr auto names() { return NAMES("get_txpool_backlog"); }
-  };
-
   //-----------------------------------------------
   /// Get the transaction pool statistics.
   ///
@@ -2733,7 +2710,6 @@ namespace cryptonote::rpc {
     GETBLOCKCOUNT,
     MINING_STATUS,
     GET_TRANSACTION_POOL_HASHES,
-    GET_TRANSACTION_POOL_BACKLOG,
     GET_TRANSACTION_POOL_STATS,
     GET_TRANSACTIONS,
     GET_SERVICE_NODES,