Drop outdated attention requested related db fields

We no longer need state for merge_request_assignees and update_state_by_user_id for both merge_request_assignees and merge_request_reviewers.

We should follow the steps from https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#dropping-columns

• Ignoring the column (15.4) - !96324 (merged)
• Dropping the column (15.5) - !98298 (merged), !98299 (merged)
• Removing the ignore rule (15.6) - !102136 (merged)

Related to #366442 (closed)