Remove index_notes_on_noteable_id_noteable_type_and_id index async

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Background

Refactor lazy_user_notes and replace db index (!185362 - closed) introduced an aynsc index to notes table and will be created sync with Create index index_notes_on_noteable_id_noteabl... (#526558 - closed)

Once that is done, the index_notes_on_noteable_id_noteable_type_and_id will be covered by the new index and it can be removed

Proposal

Follow https://docs.gitlab.com/development/database/adding_database_indexes/#drop-indexes-asynchronously to drop the index asynchronously. This issue will track the prepare step to drop it asynchronously