remove unnecessary mutex acquisition in notfound loop

Previously, we accessed m_txdownloadman within the loop. Now, we are
just checking and making copies of a local variable.

src/net_processing.cpp

@@ -4871,7 +4871,6 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
         vRecv >> vInv;
         std::vector<uint256> tx_invs;
         if (vInv.size() <= node::MAX_PEER_TX_ANNOUNCEMENTS + MAX_BLOCKS_IN_TRANSIT_PER_PEER) {
-            LOCK(m_tx_download_mutex);
             for (CInv &inv : vInv) {
                 if (inv.IsGenTxMsg()) {
                     // If we receive a NOTFOUND message for a tx we requested, mark the announcement for it as