No longer gate personal snippet reference filters on FF

What does this MR do and why?

Remove the personal_snippet_reference_filters FF and make it official!

References

• [Feature flag] Rollout of `personal_snippet_ref... (#235155 - closed)
• Personal Snippet URLs aren't replaced by their ... (#217306 - closed)
  • Add support for a user parent resource in GFM (!38571 - merged)
  • Add PersonalSnippetReferenceFilter for personal... (!233735 - merged)
  • Document personal snippet references (!240795 - merged)
• Refactor preview markdown for PersonalSnippets (#232950 - closed)
• Support GFM references for PersonalSnippets (gitlab-org#4185 - closed)

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