pg_dump: Remove trivial usage of PQExpBuffer

I've been looking at ways to reorganize and/or clean up pg_dump.c.

One thing I have noticed is the usage of PQExpBuffer in situations where
the query has no optional parts and no string interpolation.

Attached is a patch to replace those usages with the string literal itself.
There are still a few cases where a buffer is used for a trivial query and
then reset and reused for a more complicated query generation. In those
cases, I did not make the change so as to keep this patch simple.