p2p: remove nil guard (#4901)

https://github.com/tendermint/tendermint/pull/4893#discussion_r430940912

p2p/switch.go

@@ -320,9 +320,6 @@ func (sw *Switch) Peers() IPeerSet {
 // TODO: make record depending on reason.
 func (sw *Switch) StopPeerForError(peer Peer, reason interface{}) {
 	sw.Logger.Error("Stopping peer for error", "peer", peer, "err", reason)
-	if peer == nil {
-		return
-	}
 	sw.stopAndRemovePeer(peer, reason)
 
 	if peer.IsPersistent() {