Remove unused spec shared example for snippets

What does this MR do?

This MR removes an unused shared spec example for Snippet requests.

I've searched the codebase and can find no use of it and we have similar test coverage already in place within the Snippet entity specs https://gitlab.com/gitlab-org/gitlab/-/blob/master/spec/lib/api/entities/snippet_spec.rb#L38

I have not added a changelog due to this being a developer facing change

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.