Avoid extra AggCheckCallContext() checks in ordered-set aggregates.

In the transition functions, we don't really need to recheck this after the
first call.  I had been feeling paranoid about possibly getting a non-null
argument value in some other context; but it's probably game over anyway
if we have a non-null "internal" value that's not what we are expecting.

In the final functions, the general convention in pre-existing final
functions seems to be that an Assert() is good enough, so do it like that
here too.

This seems to save a few tenths of a percent of overall query runtime,
which isn't much, but still it's just overhead if there's not a plausible
case where the checks would fire.