Updated the following files in accordance to issue #316

Updated src/postorius /forms/list_forms.py , /test/test_list_forms.py , tests/mailman_api_tests/test_list_settings.py , tests/mailman_api_tests/test_user.py

These settings are obsolete as the welcome and good bye message templates can now be set via the Templates page, and the settings should probably be removed.