remove context manager API from internal class

... will return in another PR

sdk/identity/azure-identity/azure/identity/_credentials/client_assertion.py

@@ -34,18 +34,6 @@ def __init__(self, tenant_id, client_id, get_assertion, **kwargs):
         self._client = AadClient(tenant_id, client_id, **kwargs)
         super(ClientAssertionCredential, self).__init__(**kwargs)
 
-    def __enter__(self):
-        self._client.__enter__()
-        return self
-
-    def __exit__(self, *args):
-        self._client.__exit__(*args)
-
-    def close(self):
-        # type: () -> None
-        """Close the credential's transport session."""
-        self.__exit__()
-
     def _acquire_token_silently(self, *scopes, **kwargs):
         # type: (*str, **Any) -> Optional[AccessToken]
         return self._client.get_cached_access_token(scopes, **kwargs)