Clean up Snippet controllers

What does this MR do?

Removes unneccessary actions, routes, JSON and blob handling in the Snippet controllers, now that the snippets_vue FF has been removed and they're no longer used.

Refs #231285 (closed)

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