Drop merge_requests trigram indexes for all instances

Sincheol (David) Kimrequested to merge
554568-remove-title-and-description-gin-indexes-from-merge-requests-all-instances into master

What does this MR do and why?

As discussed in https://gitlab.com/gitlab-org/gitlab/-/issues/554568#note_2663305881, we don't think these title and description trigram indexes are used in realistic queries and should be removed.

We've already dropped it for .com due to the high WAL rate, but we should also drop it for other instances.

References

#554568

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