revert(tts): remove pcm_24000 encoding, keep language=de

pcm_24000 caused silent playback through livekit. Reverting to
plugin default encoding which is known working.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

voice.py

@@ -594,7 +594,6 @@ class VoiceSession:
                 tts=elevenlabs.TTS(
                     voice_id=voice_id,
                     model="eleven_multilingual_v2",
-                    encoding="pcm_24000",
                     language="de",
                     api_key=ELEVENLABS_KEY,
                     http_session=self._http_session,