Remove unused has_admin_extensions()

has_admin_extensions() had not been used since the commit
Id36a6ebddb618a78cee7025c9537cd1e2746190e.
So it is nice to remove it for cleanup.

Change-Id: Ie828bb82da4a6453f8934a8a85a23944bc4f4eed
diff --git a/tempest/services/identity/v2/json/identity_client.py b/tempest/services/identity/v2/json/identity_client.py
index f37bc08..2d71359 100644
--- a/tempest/services/identity/v2/json/identity_client.py
+++ b/tempest/services/identity/v2/json/identity_client.py
@@ -27,21 +27,6 @@
         body = json.loads(body)
         return service_client.ResponseBody(resp, body)
 
-    def has_admin_extensions(self):
-        """
-        Returns True if the KSADM Admin Extensions are supported
-        False otherwise
-        """
-        if hasattr(self, '_has_admin_extensions'):
-            return self._has_admin_extensions
-        # Try something that requires admin
-        try:
-            self.list_roles()
-            self._has_admin_extensions = True
-        except Exception:
-            self._has_admin_extensions = False
-        return self._has_admin_extensions
-
     def create_role(self, name):
         """Create a role."""
         post_body = {