[trivial] Remove misleading 'const'

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.

src/txmempool.h

@@ -714,7 +714,7 @@ public:
     void RemoveUnbroadcastTx(const uint256& txid, const bool unchecked = false);
 
     /** Returns transactions in unbroadcast set */
-    const std::set<uint256> GetUnbroadcastTxs() const {
+    std::set<uint256> GetUnbroadcastTxs() const {
         LOCK(cs);
         return m_unbroadcast_txids;
     }