Obsolete reference to _bt_tuplecompare() within tuplesort.c

I found a reference made obsolete by commit 9e85183b, which is from
way back in 2000.

comparetup_index_btree() says:

/*
* This is similar to _bt_tuplecompare(), but we have already done the
* index_getattr calls for the first column, and we need to keep track of
* whether any null fields are present.  Also see the special treatment
* for equal keys at the end.
*/

I think that this comment should simply indicate that the routine is
similar to comparetup_heap(), except that it takes care of the special
tie-break stuff for B-Tree sorts, as well as enforcing uniqueness
during unique index builds. It should not reference _bt_compare() at
all (which is arguably the successor to _bt_tuplecompare()), since
_bt_compare() is concerned with a bunch of stuff highly specific to
the B-Tree implementation (e.g. having a hard-wired return value for
comparisons involving the first data item on an internal page).

-- 
Peter Geoghegan