Remove read_file/write_file tests. These were originally intended to

*fail*, to test that plpython didn't allow untrusted operations.
When we changed plpython to plpythonu because python didn't actually have
a secure sandbox mode, someone (probably me :-() misinterpreted the tests
as checking whether Python's file I/O works.  Which is a stupid thing for
us to be testing.  Remove it so we don't clutter the filesystem with
random temporary files.