Remove a redundant test

[kitchoi]

This PR removes a test that demonstrates the (unexpected) behaviour noted in #194.

The test actually fails with Traits 6.0 because the particular setup of List of List happened to work there:

======================================================================
FAIL: test_nested_container_mutation_not_supported (apptools.preferences.tests.test_preferences_helper.PreferencesHelperTestCase)
Known limitation: mutation on nested containers are not
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/kchoi/Work/ETS/apptools/apptools/preferences/tests/test_preferences_helper.py", line 316, in test_nested_container_mutation_not_supported
    self.assertEqual(
AssertionError: AssertionError not raised

There is no much apptools can do to fix that. The test is closer to testing the limitations of Traits rather than testing Apptools itself, hence it should be removed.

Checklist

• Add a news fragment if this PR is news-worthy for end users. (see docs/releases/README.rst): The removed test had not been released in the past.

[aaronayres35]

LGTM!
Maybe we will want to re-add such a test when (if?) we drop compatibility with traits 6.0