fix: remove disallowed configuration value

Author: engelke

dialogflow/participant_management.py

@@ -160,7 +160,6 @@ def request_generator(audio_config, audio_file_path):
         audio_encoding=audio_encoding,
         language_code="en-US",
         sample_rate_hertz=sample_rate_hertz,
-        single_utterance=True,
         model="phone_call",
         # Make sure your project is Dialogflow ES ENTERPRISE_TIER in order to "USE_ENHANCED" model.
         model_variant="USE_ENHANCED",