mrhlpr.mr: Remove None default value for origin in get_artifacts_zip

It makes no sense to allow None here given that the first usage of
origin is accessing a property of it with the dot operator, an operation
which would be guaranteed to fail on a None value.

mrhlpr/mr.py

@@ -119,7 +119,7 @@ def get_status(
     )
 
 
-def get_artifacts_zip(mr_id, no_cache=False, origin=None):
+def get_artifacts_zip(mr_id, no_cache=False, origin):
     """Download artifacts from the GitLab API.
 
     :param mr_id: merge request ID