Fix lost requests on disconnect need_confirmation.clear

The pending acks shouldn't be cleared, in case of a reconnection
these would be pretty common. E.g. disconnect(), connect(),
invoke, repeat.

telethon/network/mtproto_sender.py

@@ -79,7 +79,6 @@ class MtProtoSender:
         """Disconnects from the server."""
         __log__.info('Disconnecting MtProtoSender...')
         self.connection.close()
-        self._need_confirmation.clear()
         self._clear_all_pending()
 
     # region Send and receive