fix: Make repolib's perform() return None instead of 0

Originally fixed through ENT-5549, we need this patch in RHEL 8 as well.

(Cherry-picked from bdb6a07)

src/subscription_manager/repolib.py

@@ -411,7 +411,7 @@ def perform(self):
                     log.info("Removing %s due to manage_repos configuration." %
                             repo_file.path)
             RepoActionInvoker.delete_repo_file()
-            return 0
+            return None
 
         repo_pairs = []
         for repo_class, server_val_repo_class in get_repo_file_classes():