Minor cleanup/optimization in pg_dump.

Minor cleanup/optimization in pg_dump.

In the wake of commits 05649b88c and 5209c0ba0, findComments() and
findSecLabels() no longer use their "Archive *fout" arguments,
so get rid of those.

While doing that, I noticed that there's no very good reason why
dumpCompositeTypeColComments() should be doing its own query to fetch
the column names of the composite type, when the calling function has
just fetched the same data.  Tweak it to use that query result.  This
probably doesn't save a lot for most people, because since 5209c0ba0
we won't get into this code at all unless the composite type has at
least one comment.  Nonetheless, it's a wasted query.