Add remove_concurrent_index to database helper

...leased/add_remove_concurrent_index_to_database_helper.yml

...te/20160615142710_add_index_on_requested_at_to_members.rb

...20160616103005_remove_keys_fingerprint_index_if_exists.rb

...te/20160616103948_add_unique_index_to_keys_fingerprint.rb

db/migrate/20160620115026_add_index_on_runners_locked.rb

db/migrate/20160715134306_add_index_for_pipeline_user_id.rb

db/migrate/20160725104020_merge_request_diff_remove_uniq.rb

db/migrate/20160725104452_merge_request_diff_add_index.rb

.../20160802010328_remove_builds_enable_index_on_projects.rb

...rate/20160803161903_add_unique_index_to_lists_label_id.rb

db/migrate/20160805041956_add_deleted_at_to_namespaces.rb

db/migrate/20160808085602_add_index_for_build_token.rb

db/migrate/20160810142633_remove_redundant_indexes.rb

db/migrate/20160819221631_add_index_to_note_discussion_id.rb

...grate/20160819232256_add_incoming_email_token_to_users.rb

db/migrate/20160919145149_add_group_id_to_labels.rb

db/migrate/20160920160832_add_index_to_labels_title.rb

db/migrate/20161017125927_add_unique_index_to_labels.rb

...0161020083353_add_pipeline_id_to_merge_request_metrics.rb

...grate/20161031181638_add_unique_index_to_subscriptions.rb