Fix backfills of super large manifests by stripping metadata from all but the final layer

This is semantically valid because Docker only uses the leaf layer as the image config when reading a V2_1 manifest

Fixes https://jira.coreos.com/browse/QUAY-1351

data/registry_model/registry_pre_oci_model.py

@@ -473,8 +473,6 @@ class PreOCIModel(SharedModel, RegistryDataInterface):
     assert not tag_obj.hidden
 
     repo = tag_obj.repository
-    namespace_name = repo.namespace_user.username
-    repo_name = repo.name
 
     # Write the manifest to the DB.
     manifest = self._build_manifest_for_legacy_image(tag_obj.name, tag_obj.image)