[cleanup] Remove unused param keys (#4067)

ml-agents/mlagents/trainers/ppo/trainer.py

         super(PPOTrainer, self).__init__(
             brain_name, trainer_settings, training, run_id, reward_buff_cap
         )
-        self.param_keys = [
-            "batch_size",
-            "beta",
-            "buffer_size",
-            "epsilon",
-            "hidden_units",
-            "lambd",
-            "learning_rate",
-            "max_steps",
-            "normalize",
-            "num_epoch",
-            "num_layers",
-            "time_horizon",
-            "sequence_length",
-            "summary_freq",
-            "use_recurrent",
-            "memory_size",
-            "output_path",
-            "reward_signals",
-        ]
         self.hyperparameters: PPOSettings = cast(
             PPOSettings, self.trainer_settings.hyperparameters
         )

ml-agents/mlagents/trainers/trainer/trainer.py

         :str run_id: The identifier of the current run
         :int reward_buff_cap:
         """
-        self.param_keys: List[str] = []
         self.brain_name = brain_name
         self.run_id = run_id
         self.trainer_settings = trainer_settings