[ticket/13315] Fix loss of CAPTCHA plugin config value.

The plugins were moved to phpbb/captcha/plugins/ after this migration was created, thus the check is no longer valid and ends up resetting the value prematurely. We check for the existence of the phpbb/captcha/plugins directory, in case the user fails to remove the includes/captcha/plugins/ directory.

[PHPBB3-13315](https://tracker.phpbb.com/browse/PHPBB3-13315)