Remove some unnecessary tests of pgstat_track_counts.

We may as well make pgstat_count_heap_scan() and related macros just count
whenever rel->pgstat_info isn't null.  Testing pgstat_track_counts buys
nothing at all in the normal case where that flag is ON; and when it's OFF,
the pgstat_info link will be null, so it's still a useless test.

This change is unlikely to buy any noticeable performance improvement,
but a cycle shaved is a cycle earned; and my investigations earlier today
convinced me that we're down to the point where individual instructions in
the inner execution loops are starting to matter.