p2p: Avoid an IsAncestorOfBestHeaderOrTip call

Just don't call this function when it won't have any effect.

Note that we can't remove the LookupBlockIndex call, since `last_received_header`
is needed to check if new headers were received (`received_new_header`).

src/net_processing.cpp

@@ -2919,9 +2919,7 @@ void PeerManagerImpl::ProcessHeadersMessage(CNode& pfrom, Peer& peer,
     {
         LOCK(cs_main);
         last_received_header = m_chainman.m_blockman.LookupBlockIndex(headers.back().GetHash());
-        if (IsAncestorOfBestHeaderOrTip(last_received_header)) {
+        already_validated_work = already_validated_work || IsAncestorOfBestHeaderOrTip(last_received_header);
-            already_validated_work = true;
-        }
     }
 
     // If our peer has NetPermissionFlags::NoBan privileges, then bypass our