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.

src/pl/plpython/sql/plpython_error.sql

@@ -7,7 +7,3 @@ SELECT invalid_type_uncaught('rick');
 SELECT invalid_type_caught('rick');
 SELECT invalid_type_reraised('rick');
 SELECT valid_type('rick');
-
--- Security sandbox tests
-SELECT write_file('/tmp/plpython','Only trusted users should be able to do this!');
-SELECT read_file('/tmp/plpython');