Remove threaded from trainer_controller

ml-agents/mlagents/trainers/tests/test_simple_rl.py

             sampler_manager=SamplerManager(None),
             resampling_interval=None,
             save_freq=save_freq,
-            threaded=False,
         )
 
         # Begin training

ml-agents/mlagents/trainers/trainer_controller.py

         training_seed: int,
         sampler_manager: SamplerManager,
         resampling_interval: Optional[int],
-        threaded: bool = True,
     ):
         """
         :param model_path: Path to save the model.
         self.sampler_manager = sampler_manager
         self.resampling_interval = resampling_interval
 
-        self.threaded = threaded
         self.trainer_threads: List[threading.Thread] = []
         self.kill_trainers = False
         np.random.seed(training_seed)