Remove bogus const qualifier on PageGetItem() argument

Remove bogus const qualifier on PageGetItem() argument

The function ends up casting away the const qualifier, so it was a
lie.  No callers appear to rely on the const qualifier on the
argument, so the simplest solution is to just remove it.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/beusplf77varvhip6ryuhd2fchsx26qmmhduqz432bnglq634b%402dx4k6yxj4cm