remove loggerDict.clear() from tearDown method, fixes #3805

It causes problems with the new caching in the py37 logger module.

Removing loggerDict.clear() fixes this and makes the tests work again.
Also, it does not seem to have any negative effect, neither on py36
nor on py37.

See also: https://bugs.python.org/issue34269

src/borg/testsuite/archiver.py

@@ -279,8 +279,6 @@ class ArchiverTestCaseBase(BaseTestCase):
         os.chdir(self._old_wd)
         # note: ignore_errors=True as workaround for issue #862
         shutil.rmtree(self.tmpdir, ignore_errors=True)
-        # destroy logging configuration
-        logging.Logger.manager.loggerDict.clear()
         setup_logging()
 
     def cmd(self, *args, **kw):