Always drop the least preferred HB peer when adding a new one.

When a BIP152 HB-mode peer is in the least preferred position and
 disconnects, they will not be by ForNode on the next loop. They
 will continue to sit in that position and prevent deactivating
 HB mode for peers that are still connected.

There is no reason for them to stay in the list if already gone,
 so drop the first element unconditionally if there are too many.

Fixes issue bitcoin#9163.

src/main.cpp

@@ -529,12 +529,11 @@ void MaybeSetPeerAsAnnouncingHeaderAndIDs(const CNodeState* nodestate, CNode* pf
         if (lNodesAnnouncingHeaderAndIDs.size() >= 3) {
             // As per BIP152, we only get 3 of our peers to announce
             // blocks using compact encodings.
-            bool found = connman.ForNode(lNodesAnnouncingHeaderAndIDs.front(), [&connman, fAnnounceUsingCMPCTBLOCK, nCMPCTBLOCKVersion](CNode* pnodeStop){
+            connman.ForNode(lNodesAnnouncingHeaderAndIDs.front(), [&connman, fAnnounceUsingCMPCTBLOCK, nCMPCTBLOCKVersion](CNode* pnodeStop){
                 connman.PushMessage(pnodeStop, NetMsgType::SENDCMPCT, fAnnounceUsingCMPCTBLOCK, nCMPCTBLOCKVersion);
                 return true;
             });
-            if(found)
-                lNodesAnnouncingHeaderAndIDs.pop_front();
+            lNodesAnnouncingHeaderAndIDs.pop_front();
         }
         fAnnounceUsingCMPCTBLOCK = true;
         connman.PushMessage(pfrom, NetMsgType::SENDCMPCT, fAnnounceUsingCMPCTBLOCK, nCMPCTBLOCKVersion);