In 10.3, all records will be processed by purge due to MDEV-12288.
But, the insert undo records do not contain a transaction identifier.

row_purge_parse_undo_rec(): Use node->trx_id=TRX_ID_MAX for the
insert undo records. We cannot skip table lookups for these records
after DISCARD TABLESPACE other than by 'detaching' the table from
the undo logs by updating SYS_TABLES.ID on both DISCARD TABLESPACE
and IMPORT TABLESPACE.

Also, remove a redundant condition that was introduced
in the merge commit 814205f3.