[mle] remove redundant mAttachTimer stop (#3537)

The call to `SetStateDetached()` will stop the attach timer.

src/core/thread/mle.cpp

@@ -3360,7 +3360,7 @@ otError Mle::HandleChildIdResponse(const Message &aMessage, const Ip6::MessageIn
 #endif
 
     // Parent Attach Success
-    mAttachTimer.Stop();
+
     SetStateDetached();
 
     SetLeaderData(leaderData.GetPartitionId(), leaderData.GetWeighting(), leaderData.GetLeaderRouterId());