Remove Snippets#show JSON format handling

What does this MR do?

Now that Snippets use Vue on the frontend, we have cleaned up the controllers to remove unused actions (#231285 (closed)).

This MR is a follow up that removes JSON handling for snippets#show because it is no longer used.

Refs #263467 (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.