afform_clear() - Fix error when running on non-cached container

afform_clear() - Fix error when running on non-cached container

After switching the tests from an effective policy of `CIVICRM_CONTAINER_CACHE=auto`
to an effective policy of `CIVICRM_CONTAINER_CACHE=never`, this started to fail:

```
1) api_v4_AfformTest::testGetUpdateRevert with data set #0 ('mockPage', array('', '', 'civicrm/mock-page', 'access Foobar'))
Symfony\Component\DependencyInjection\Exception\BadMethodCallException: Setting service "angular" for an unknown or non-synthetic service definition on a compiled container is not allowed.

/home/me/bknix/build/dmaster/web/sites/all/modules/civicrm/vendor/symfony/dependency-injection/ContainerBuilder.php:532
/home/me/bknix/build/dmaster/web/sites/all/modules/civicrm/ext/afform/core/afform.php:520
/home/me/bknix/build/dmaster/web/sites/all/modules/civicrm/ext/afform/core/Civi/Api4/Afform.php:99
/home/me/bknix/build/dmaster/web/sites/all/modules/civicrm/Civi/Api4/Generic/BasicBatchAction.php:88
/home/me/bknix/build/dmaster/web/sites/all/modules/civicrm/Civi/Api4/Generic/BasicBatchAction.php:68
/home/me/bknix/build/dmaster/web/sites/all/modules/civicrm/Civi/Api4/Provider/ActionObjectProvider.php:68
/home/me/bknix/build/dmaster/web/sites/all/modules/civicrm/Civi/API/Kernel.php:150
/home/me/bknix/build/dmaster/web/sites/all/modules/civicrm/Civi/Api4/Generic/AbstractAction.php:238
/home/me/bknix/build/dmaster/web/sites/all/modules/civicrm/ext/afform/mock/tests/phpunit/api/v4/AfformTest.php:53
/home/me/bknix/extern/phpunit6/phpunit6.phar:570
```