Remove unused is_enabled()

is_enabled() is not used at all, and the methods which don't send
a REST request should not be included in service clients.
Then this patch removes is_enabled().

Partially implements blueprint consistent-service-method-names

Change-Id: Ia768b019158230688d19c9e95ae382523fc24967
diff --git a/tempest/services/compute/json/extensions_client.py b/tempest/services/compute/json/extensions_client.py
index cd7ecbc..ec60872 100644
--- a/tempest/services/compute/json/extensions_client.py
+++ b/tempest/services/compute/json/extensions_client.py
@@ -28,11 +28,6 @@
         self.validate_response(schema.list_extensions, resp, body)
         return service_client.ResponseBodyList(resp, body['extensions'])
 
-    def is_enabled(self, extension):
-        extensions = self.list_extensions()
-        exts = extensions['extensions']
-        return any([e for e in exts if e['name'] == extension])
-
     def show_extension(self, extension_alias):
         resp, body = self.get('extensions/%s' % extension_alias)
         body = json.loads(body)