Remove unused db fields from assignees

What does this MR do and why?

This MR removes db columns from merge_request_assignees that are no longer used since attention request feature has been removed. These fields have been set to be ignored since !96324 (merged) (merged)

Related to #371476 (closed)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #371476 (closed)