vacuumlazy.c: Remove obsolete num_tuples field.

vacuumlazy.c: Remove obsolete num_tuples field.

Commit 49c9d9fc unified VACUUM VERBOSE and autovacuum logging.  It
neglected to remove an old vacrel field that was only used by the old
VACUUM VERBOSE, so remove it now.

The previous num_tuples approach doesn't seem to have any real advantage
over the approach VACUUM VERBOSE takes now (also the approach used by
the autovacuum logging code), which is to show new_rel_tuples.
new_rel_tuples is the possibly-estimated total number of tuples left in
the table, whereas num_tuples meant the number of tuples encountered
during the VACUUM operation, after pruning, without regard for tuples
from pages skipped via the visibility map.

In passing, reorder a related vacrel field for consistency.