refactor(user_mute): remove unnecessary _bot_speaking assignment in _handle_bot_stopped_speaking

The _bot_speaking flag does not need to be set in this method,
so the redundant assignment has been removed.

src/pipecat/turns/user_mute/mute_until_first_bot_complete_user_mute_strategy.py

@@ -51,6 +51,5 @@ class MuteUntilFirstBotCompleteUserMuteStrategy(BaseUserMuteStrategy):
         return not self._first_speech_handled
 
     async def _handle_bot_stopped_speaking(self, frame: BotStoppedSpeakingFrame):
-        self._bot_speaking = False
         if not self._first_speech_handled:
             self._first_speech_handled = True