Remove index_notes_on_noteable_id_noteable_type_and_id index sync

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Background

Remove index_notes_on_noteable_id_noteable_type... (#526559) scheduled the index_notes_on_noteable_id_noteable_type_and_id index for removal

Proposal

Follow https://docs.gitlab.com/development/database/adding_database_indexes/#drop-indexes-asynchronously to drop the index synchronously.