Remove obsolete _bt_compare() comment.

btbuild() has nothing to say about how NULL values compare in nbtree.
Besides, there are _bt_compare() header comments that describe how NULL
values are handled.

src/backend/access/nbtree/nbtsearch.c

@@ -601,7 +601,6 @@ _bt_compare(Relation rel,
 
 		datum = index_getattr(itup, scankey->sk_attno, itupdesc, &isNull);
 
-		/* see comments about NULLs handling in btbuild */
 		if (scankey->sk_flags & SK_ISNULL)	/* key is NULL */
 		{
 			if (isNull)