remove unnecessary and potentially harmful check from heartbeat

- the check is unnecessary because peers emit PRUNE on Leave
- the check is harmful, because the ANNOUNCE message might have
  benn lost (or reordered after the GRAFT depending on the retry
  strategy), which would leave the mesh in an inconsistent state.

gossipsub.go

@@ -334,14 +334,6 @@ func (gs *GossipSubRouter) heartbeat() {
 	// maintain the mesh for topics we have joined
 	for topic, peers := range gs.mesh {
 
-		// check whether our peers are still in the topic
-		for p := range peers {
-			_, ok := gs.p.topics[topic][p]
-			if !ok {
-				delete(peers, p)
-			}
-		}
-
 		// do we have enough peers?
 		if len(peers) < GossipSubDlo {
 			ineed := GossipSubD - len(peers)