remove obsolete param from docstring (#3317)

ml-agents-envs/mlagents_envs/environment.py

         :bool docker_training: Informs this class whether the process is being run within a container.
         :bool no_graphics: Whether to run the Unity simulator in no-graphics mode
         :int timeout_wait: Time (in seconds) to wait for connection from environment.
-        :bool train_mode: Whether to run in training mode, speeding up the simulation, by default.
         :list args: Addition Unity command line arguments
         :list side_channels: Additional side channel for no-rl communication with Unity
         """