Clean up after bigint conversion for notes and dependent tables for GitLab.com

This is the next step after Swap integer and bigint columns for notes and d... (#392815 - closed).

Following the process outlined here, we have to remove the triggers, and drop the old integer columns for notes and all related tables.

This migrations will be done for GitLab.com only, as we decided to let self-manages instances complete the back-fill migrations till the required stop 15.11, and have not yet swapped columns for them.

The columns to clean up are:

• notes.id
• todos.note_id
• system_note_metadata.note_id
• epic_user_mentions.note_id
• suggestions.note_id
• issue_user_mentions.note_id
• note_diff_files.diff_note_id
• snippet_user_mentions.note_id
• design_user_mentions.note_id
• vulnerability_user_mentions.note_id
• commit_user_mentions.note_id
• merge_request_user_mentions.note_id
• timelogs.note_id
• award_emoji.awardable_id
• events.target_id