[trivial] Remove misleading 'const'

Summary:
Returning by const value is only meaningful in a specific circumstance around
user defined types. In this case, the const is not enforcing any restrictions
on the call site, so is misleading.

This is a backport of Core [[bitcoin/bitcoin#18807 | PR18807]] [7/9]
bitcoin/bitcoin@750456d

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9034

src/txmempool.h

@@ -796,7 +796,7 @@ class CTxMemPool {
     void RemoveUnbroadcastTx(const TxId &txid, const bool unchecked = false);
 
     /** Returns transactions in unbroadcast set */
-    const std::set<TxId> GetUnbroadcastTxs() const {
+    std::set<TxId> GetUnbroadcastTxs() const {
         LOCK(cs);
         return m_unbroadcast_txids;
     }