[mempool] Revert unbroadcast set to tracking just txid

When I originally implemented the unbroadcast set in 18038, it just tracked
txids. After 18038 was merged, I offered a patch to 18044 to make the
unbroadcast changes compatible with wtxid relay. In this patch, I updated
`unbroadcast_txids` to a map of txid -> wtxid. Post merge review comments shed
light on the fact that this update was unnecessary, and distracting. So, this
commit updates the unbroadcast ids back to a set.

src/node/transaction.cpp

@@ -80,7 +80,7 @@ TransactionError BroadcastTransaction(NodeContext& node, const CTransactionRef t
     if (relay) {
         // the mempool tracks locally submitted transactions to make a
         // best-effort of initial broadcast
-        node.mempool->AddUnbroadcastTx(hashTx, tx->GetWitnessHash());
+        node.mempool->AddUnbroadcastTx(hashTx);
 
         LOCK(cs_main);
         RelayTransaction(hashTx, tx->GetWitnessHash(), *node.connman);