Drop sequences for some ID columns in MRCM table

What does this MR do and why?

These columns were created using bigserial but they shouldn't be. So this migration removes the sequences for those tables.

Nothing is writing to this table yet.

This follows https://docs.gitlab.com/development/migration_style_guide/#dropping-a-sequence.

References

https://gitlab.com/gitlab-org/gitlab/-/issues/527226

MR acceptance checklist

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Related to #527226