Remove optimization whereby parser would make only one sort-list entry

Remove optimization whereby parser would make only one sort-list entry
when two equal() targetlist items were to be added to an ORDER BY or
DISTINCT list.  Although indeed this would make sorting fractionally
faster by sometimes saving a comparison, it confuses the heck out of
later stages of processing, because it makes it look like the user
wrote DISTINCT ON rather than DISTINCT.  Bug reported by joe@piscitella.com.