Suppress MergeableSelector warning candidates in EE-only files

What does this MR do?

Suppress MergeableSelector warnings that would appear after gitlab-org/gitlab-ce!12810 merged and then ported from CE to EE.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

These EE-specific MergeableSelector warnings are blocking to merge gitlab-org/gitlab-ce!12810.

Screenshots (if relevant)

n/a

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• [n/a] Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • [n/a] Has been reviewed by Backend
  • [n/a] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #3879 (closed)