rm unnecessary __contains__ duplicate code (#1147)

`contains` really should not exist when it does exactly what the builtin
magic method `__contains__` was meant for, but that would break backward
compatibility.

gym/spaces/box.py

@@ -38,8 +38,6 @@ class Box(gym.Space):
     def contains(self, x):
         return x.shape == self.shape and (x >= self.low).all() and (x <= self.high).all()
 
-    __contains__ = contains
-
     def to_jsonable(self, sample_n):
         return np.array(sample_n).tolist()
     def from_jsonable(self, sample_n):