Remove obsolate comments from 047_checkpoint_physical_slot

Dear hackers,
(CC: Alexander, who is an original committer)

While reviewing others, I found $SUBJECT.
Initially the tests inserted 2M tuples twice, and 4464fddf improves to use the advance_wal().
However, code comments in the test missed to be updated.
PSA the fix patch.

This exists PG17+, which has the same TAP test.

Best regards,
Hayato Kuroda
FUJITSU LIMITED