Filtered output cleanup in expression dumping

Filtered output cleanup in expression dumping

Most of the expression-dumping code uses filtered output, but a few
functions did not.  This patch cleans up these instance.

Note that this won't cause any behavior change, because the only calls
to dump_prefix_expression pass in gdb_stdlog.  However, in the long
run it's easier to audit the code if the number of uses of _unfiltered
is reduced.