Remove Image join from get_active_tag

It isn't used anywhere in the query and appears to be completely unnecessary

data/model/tag.py

@@ -246,7 +246,6 @@ def store_tag_manifest(namespace, repo_name, tag_name, docker_image_id, manifest
 def get_active_tag(namespace, repo_name, tag_name):
   return _tag_alive(RepositoryTag
                     .select()
-                    .join(Image)
                     .join(Repository)
                     .join(Namespace, on=(Repository.namespace_user == Namespace.id))
                     .where(RepositoryTag.name == tag_name, Repository.name == repo_name,