Remove a deprecated attribute, scheduled for 1.21+

Actually, it was broken since MSAL 1.17 because we forgot to import warnings.
So, that makes the removal today even easier. Nobody needs it anymore.

Author: rayluo

msal/authority.py

@@ -52,12 +52,6 @@ class Authority(object):
     """
     _domains_without_user_realm_discovery = set([])
 
-    @property
-    def http_client(self):  # Obsolete. We will remove this eventually
-        warnings.warn(
-            "authority.http_client might be removed in MSAL Python 1.21+", DeprecationWarning)
-        return self._http_client
-
     def __init__(
             self, authority_url, http_client,
             validate_authority=True,