[refactor] Simplify check for block-relay-only connection.

Previously we deduced it was a block-relay-only based on presence of the
m_tx_relay structure. Now we have the ability to identify it directly via a
connection type accessor function.

src/net_processing.cpp

@@ -2018,7 +2018,7 @@ static void ProcessHeadersMessage(CNode& pfrom, CConnman& connman, ChainstateMan
             }
         }
 
-        if (!pfrom.fDisconnect && pfrom.IsOutboundOrBlockRelayConn() && nodestate->pindexBestKnownBlock != nullptr && pfrom.m_tx_relay != nullptr) {
+        if (!pfrom.fDisconnect && pfrom.IsFullOutboundConn() && nodestate->pindexBestKnownBlock != nullptr) {
             // If this is an outbound full-relay peer, check to see if we should protect
             // it from the bad/lagging chain logic.
             // Note that block-relay-only peers are already implicitly protected, so we