mempool: codify existing assumption about duplicate txids during removal

Also explicitly check for txid equality rather than transaction equality as the
former is a tighter constraint if witness data is included when comparing the
full transactions.

Co-authored-by: glozow <gloriajzhao@gmail.com>

src/txmempool.cpp

@@ -652,7 +652,7 @@ void CTxMemPool::removeConflicts(const CTransaction &tx)
         auto it = mapNextTx.find(txin.prevout);
         if (it != mapNextTx.end()) {
             const CTransaction &txConflict = *it->second;
-            if (txConflict != tx)
+            if (Assume(txConflict.GetHash() != tx.GetHash()))
             {
                 ClearPrioritisation(txConflict.GetHash());
                 removeRecursive(txConflict, MemPoolRemovalReason::CONFLICT);