Remove non-working Stream __aenter__/__aexit__ methods.

Fixes adafruit#63

Author: kbsriram

asyncio/stream.py

@@ -36,12 +36,6 @@ def get_extra_info(self, v):
 
         return self.e[v]
 
-    async def __aenter__(self):
-        return self
-
-    async def __aexit__(self, exc_type, exc, tb):
-        await self.close()
-
     def close(self):
         pass