refactor: Remove unused COINBASE_FLAGS

Commit d449772 stopped setting
COINBASE_FLAGS, and it looks like it hasn't been used since P2SH.
Update the help string to remove "flags", which is not specified in
BIP 22.

src/miner.cpp

@@ -435,7 +435,7 @@ void IncrementExtraNonce(CBlock* pblock, const CBlockIndex* pindexPrev, unsigned
     ++nExtraNonce;
     unsigned int nHeight = pindexPrev->nHeight+1; // Height first in coinbase required for block.version=2
     CMutableTransaction txCoinbase(*pblock->vtx[0]);
-    txCoinbase.vin[0].scriptSig = (CScript() << nHeight << CScriptNum(nExtraNonce)) + COINBASE_FLAGS;
+    txCoinbase.vin[0].scriptSig = (CScript() << nHeight << CScriptNum(nExtraNonce));
     assert(txCoinbase.vin[0].scriptSig.size() <= 100);
 
     pblock->vtx[0] = MakeTransactionRef(std::move(txCoinbase));

src/rpc/mining.cpp

@@ -375,9 +375,7 @@ static UniValue getblocktemplate(const JSONRPCRequest& request)
             "      }\n"
             "      ,...\n"
             "  ],\n"
-            "  \"coinbaseaux\" : {                 (json object) data that should be included in the coinbase's scriptSig content\n"
-            "      \"flags\" : \"xx\"                  (string) key name is to be ignored, and value included in scriptSig\n"
-            "  },\n"
+            "  \"coinbaseaux\" : { ... },            (json object) data that should be included in the coinbase's scriptSig content\n"
             "  \"coinbasevalue\" : n,              (numeric) maximum allowable input to coinbase transaction, including the generation award and transaction fees (in satoshis)\n"
             "  \"coinbasetxn\" : { ... },          (json object) information for coinbase transaction\n"
             "  \"target\" : \"xxxx\",                (string) The hash target\n"
@@ -607,7 +605,6 @@ static UniValue getblocktemplate(const JSONRPCRequest& request)
     }
 
     UniValue aux(UniValue::VOBJ);
-    aux.pushKV("flags", HexStr(COINBASE_FLAGS.begin(), COINBASE_FLAGS.end()));
 
     arith_uint256 hashTarget = arith_uint256().SetCompact(pblock->nBits);
 

src/validation.cpp

@@ -129,9 +129,6 @@ CFeeRate minRelayTxFee = CFeeRate(DEFAULT_MIN_RELAY_TX_FEE);
 CBlockPolicyEstimator feeEstimator;
 CTxMemPool mempool(&feeEstimator);
 
-/** Constant stuff for coinbase transactions we create: */
-CScript COINBASE_FLAGS;
-
 // Internal stuff
 namespace {
     CBlockIndex* pindexBestInvalid = nullptr;

src/validation.h

@@ -137,7 +137,6 @@ struct BlockHasher
     size_t operator()(const uint256& hash) const { return ReadLE64(hash.begin()); }
 };
 
-extern CScript COINBASE_FLAGS;
 extern CCriticalSection cs_main;
 extern CBlockPolicyEstimator feeEstimator;
 extern CTxMemPool mempool;