Validate fk_rails_3f92913d27 FK constraint on ci_runner_machines

Remove any orphaned records (since new orphaned records continue to be created after #503748 (closed)) before adding the FK.

Also remove runner_id from ignored_fk_columns_map in https://gitlab.com/gitlab-org/gitlab/blob/52471b5228b147df00e2c5cacdc466afe5f1cbbd/spec/db/schema_spec.rb#L119-119.