Fix #994 Remove unnecessary error for FCM (#999)

There is no need to log and error when the API returns the existing future.

Author: chkuang-g

messaging/src/ios/messaging.mm

@@ -180,7 +180,6 @@ void NotifyListenerSet(Listener* listener) {
  FIREBASE_ASSERT_RETURN(RequestPermissionLastResult(), internal::IsInitialized());
 
  if (RequestPermissionLastResult().status() == kFutureStatusPending) {
- LogError("Status is pending. Return the pending future.");
  return RequestPermissionLastResult();
  }