Remove test_fn and train_fn as they are not used in PPO PistonBall ex…

…ample for PettingZoo (#840)

Specifically, BasePolicy.set_eps seems to be a remnant from using DQN in
other examples.

* Removed unused functions (test_fn and train_fn) from the pettingzoo
example with PistonBall. These functions use set_eps which is not
available for PPO and is not even called once in the file.

test/pettingzoo/pistonball_continuous.py

@@ -237,12 +237,6 @@ def save_best_fn(policy):
     def stop_fn(mean_rewards):
         return False
 
-    def train_fn(epoch, env_step):
-        [agent.set_eps(args.eps_train) for agent in policy.policies.values()]
-
-    def test_fn(epoch, env_step):
-        [agent.set_eps(args.eps_test) for agent in policy.policies.values()]
-
     def reward_metric(rews):
         return rews[:, 0]