Remove snippets_edit_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 snippets_edit_vue feature flag and all it's references / dead code.

#220797 (closed)

Screenshots

Everything works 😄

20200930_snippets_edit_everything_works

Does this MR meet the acceptance criteria?

Conformity

• [ ] Changelog entry Not needed since this feature has been enabled by default
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content