test: remove unused testpy code

The temporary directory for tests is managed in the JS `common` module.
Remove unused variable from `testpy` that is legacy from when it was
managed in the Python test harness.

PR-URL: https://github.com/nodejs/node/pull/12844
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Cai <davidcai1993@yahoo.com>

test/testpy/__init__.py

@@ -44,7 +44,6 @@ class SimpleTestCase(test.TestCase):
     self.config = config
     self.arch = arch
     self.mode = mode
-    self.tmpdir = join(dirname(self.config.root), 'tmp')
     if additional is not None:
       self.additional_flags = additional
     else: