The rollback of the modification of a pre-existing record
should involve a purge-like operation. Before MDEV-12288
the only purge-like operation was the removal of a
delete-marked record.

After MDEV-12288, any rollback of updating an existing record
must reset the DB_TRX_ID column when it is no longer visible
in the purge read view.

row_vers_must_preserve_del_marked(): Remove. It is cleaner to
perform the check directly in row0umod.cc.

row_trx_id_offset(): Auxiliary function to retrieve the byte
offset of DB_TRX_ID in a clustered index leaf page record.

row_undo_mod_must_purge(): Determine if a record should be purged.

row_undo_mod_clust(): For temporary tables, skip the purge checks.
When rolling back an update so that the original record was not
delete-marked, reset DB_TRX_ID if the history is no longer visible.