I think you can get rid of SYNC_UNLINK_REQUEST, sync_unlinkfiletag,
mdunlinkfiletag as these are all now unused.
Are there any special hazards here if the plan in [1] goes ahead? If
the relfilenode allocation is logged and replayed then it should be
fine to crash and recover multiple times in a row while creating and
dropping tables, with wal_level=minimal, I think. It would be bad if
the allocator restarted from a value from the checkpoint, though.
[1]
https://www.postgresql.org/message-id/flat/CA%2BTgmoYmw%3D%3DTOJ6EzYb_vcjyS09NkzrVKSyBKUUyo1zBEaJASA%40mail.gmail.com