Remove background migrations for old schemas

On the assumption that a background migration whose specs need a schema
older than 2018 is obsoleted by this migration squash, we can remove
both specs and code for those that fail to run in CI as a result of the
schema at that date no longer existing.

This is true for all but the MigrateStageStatus background migration,
which is also used from the MigrateBuildStage background migration.

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