Combine ObjectStorage spec contexts

What does this MR do?

This MR combines ObjectStorage two spec contexts.

uploaders/object_storage_spec.rb previously had two context blocks on the same tree level that were both labelled 'when local file is used':

• https://gitlab.com/gitlab-org/gitlab/-/blob/44002f46888a5944c710a04a016f6fa3ef45e466/spec/uploaders/object_storage_spec.rb#L599
• https://gitlab.com/gitlab-org/gitlab/-/blob/44002f46888a5944c710a04a016f6fa3ef45e466/spec/uploaders/object_storage_spec.rb#L615

The decision was made in #209967 (closed) to combine them.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done