Cleanup task system note renaming background migration

What does this MR do and why?

Cleanup for ChangeTaskSystemNoteWordingToChecklistItem background migration.

1. Make sure the background migration has finished running.
2. Remove the temporary index that was introduced to make the migration faster.

Migration Output

UP

bin/rails db:migrate
main: == 20220920213504 FinalizeTaskSystemNoteRenaming: migrating ===================
main: == 20220920213504 FinalizeTaskSystemNoteRenaming: migrated (0.0382s) ==========

main: == 20220920214524 RemoveTaskSystemNoteRenameTempIndex: migrating ==============
main: -- transaction_open?()
main:    -> 0.0000s
main: -- indexes(:system_note_metadata)
main:    -> 0.0047s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0005s
main: -- remove_index(:system_note_metadata, {:algorithm=>:concurrently, :name=>"tmp_index_system_note_metadata_on_id_where_task"})
main:    -> 0.0044s
main: -- execute("RESET statement_timeout")
main:    -> 0.0004s
main: == 20220920214524 RemoveTaskSystemNoteRenameTempIndex: migrated (0.0188s) =====

ci: == 20220920213504 FinalizeTaskSystemNoteRenaming: migrating ===================
ci: -- The migration is skipped since it modifies the schemas: [:gitlab_main].
ci: -- This database can only apply migrations in one of the following schemas: [:gitlab_ci, :gitlab_shared, :gitlab_internal].
ci: == 20220920213504 FinalizeTaskSystemNoteRenaming: migrated (0.0002s) ==========

ci: == 20220920214524 RemoveTaskSystemNoteRenameTempIndex: migrating ==============
ci: -- transaction_open?()
ci:    -> 0.0000s
ci: -- indexes(:system_note_metadata)
ci:    -> 0.0063s
ci: -- execute("SET statement_timeout TO 0")
ci:    -> 0.0004s
ci: -- remove_index(:system_note_metadata, {:algorithm=>:concurrently, :name=>"tmp_index_system_note_metadata_on_id_where_task"})
ci:    -> 0.0033s
ci: -- execute("RESET statement_timeout")
ci:    -> 0.0004s
ci: == 20220920214524 RemoveTaskSystemNoteRenameTempIndex: migrated (0.0157s) =====

DOWN

bin/rails db:rollback:main STEP=2
main: == 20220920214524 RemoveTaskSystemNoteRenameTempIndex: reverting ==============
main: -- transaction_open?()
main:    -> 0.0000s
main: -- index_exists?(:system_note_metadata, [:id, :action], {:where=>"action = 'task'", :name=>"tmp_index_system_note_metadata_on_id_where_task", :algorithm=>:concurrently})
main:    -> 0.0079s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0004s
main: -- add_index(:system_note_metadata, [:id, :action], {:where=>"action = 'task'", :name=>"tmp_index_system_note_metadata_on_id_where_task", :algorithm=>:concurrently})
main:    -> 0.0083s
main: -- execute("RESET statement_timeout")
main:    -> 0.0005s
main: == 20220920214524 RemoveTaskSystemNoteRenameTempIndex: reverted (0.0260s) =====

main: == 20220920213504 FinalizeTaskSystemNoteRenaming: reverting ===================
main: == 20220920213504 FinalizeTaskSystemNoteRenaming: reverted (0.0001s) ==========

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #371410 (closed)