Snippet blob search removal

What does this MR do?

Removes remaining Snippet blob (file_name and content) searching references from the codebase and refactors the Snippet searching accordingly.

In particular, this involves changing multi-scope support to only scoping Snippet searches by snippet_titles (which also includes descriptions, issue to rename the scope is here)

As well as removing these references, this MR also removes the search_code method and elasticsearch equivalent.

It's worth noting that references in views are handled by this MR, and API references in this MR.

Relates to #208882 (closed)

I believe this falls under the category of developer-facing changes, so I haven't added a changelog entry, but happy to do so if that's incorrect!

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.