Rails5: fix delete blob

What does this MR do?

Rails5 fix for delete blob. See !19876 (merged) for details.

Fixes spec/controllers/projects/blob_controller_spec.rb:390 see https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/109556404

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides