Remove inappropriate warning (warns about supported case according to docstring)

tianshou/data/collector.py

@@ -97,7 +97,6 @@ class Collector:
     ) -> None:
         super().__init__()
         if isinstance(env, gym.Env) and not hasattr(env, "__len__"):
-            warnings.warn("Single environment detected, wrap to DummyVectorEnv.")
             self.env = DummyVectorEnv([lambda: env])
         else:
             self.env = env  # type: ignore