pg_dump: Reorganize getTables()

pg_dump: Reorganize getTables()

Along the same lines as 047329624, ed2c7f65b and daa9fe8a5, reduce
code duplication by having just one copy of the parts of the query
that are the same across all server versions; and make the
conditionals control the smallest possible amount of code.
This also gets rid of the confusing assortment of different ways
to accomplish the same result that we had here before.

While at it, make sure all three relevant parts of the function
list the fields in the same order.  This is just neatnik-ism,
of course.

Discussion: https://postgr.es/m/1240992.1634419055@sss.pgh.pa.us