Remove snippets_vue feature flag

• Step 1 - Remove snippets_edit_vue feature flag
• Step 2 - Remove snippets_vue feature flag

What does this MR do?

This MR removes the feature flag snippets_vue and the resulting dead code.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry Already enabled so changelog not necessary.
• 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.

References

• Related to #220795 (closed)