Clean up failed BackfillCiRunnerMachinesPartitionedTable BBM

BackfillCiRunnerMachinesPartitionedTable was no-oped in #520092 (closed), but the https://gitlab.com/gitlab-org/gitlab/blob/2d4cbd1aec22b4a2df3ab9d18afdbf100c878ceb/db/post_migrate/20250113163026_finalize_backfill_ci_runner_machines_partitioned_table.rb PDM is still present and referencing a no-longer existing BBM (created #534917 so we can identify such issues during MR review). After we no-op 20250113163026, we should remove that batched background migration and backport the changes to %17.9 and %17.10.