Drop title/description gin indexes from merge_requests

What does this MR do and why?

These are not performant enough to be useful and WAL rates for these two indexes are too high. It maybe still useful in self-managed instances so we only deleting it for .com for now.

Also, currently index_merge_requests_on_description_trigram and index_merge_requests_on_title_trigram take up disk space of 165GB and 49GB respectively so this will also free up some disk space.

References

#554568

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Related to #554568