Clean up silero_tts

This should only be used with --no-stream.

The shared.still_streaming implementation was faulty by design:
output_modifier should never be called when streaming is already over.

modules/shared.py

@@ -11,7 +11,6 @@ is_RWKV = False
 history = {'internal': [], 'visible': []}
 character = 'None'
 stop_everything = False
-still_streaming = False
 
 # UI elements (buttons, sliders, HTML, etc)
 gradio = {}