Remove the aggregate form of ts_rewrite(), since it doesn't work as desired

if there are zero rows to aggregate over, and the API seems both conceptually
and notationally ugly anyway.  We should look for something that improves
on the tsquery-and-text-SELECT version (which is also pretty ugly but at
least it works...), but it seems that will take query infrastructure that
doesn't exist today.  (Hm, I wonder if there's anything in or near SQL2003
window functions that would help?)  Per discussion.