[ticket/13342] Fix invalid resetting of CAPTCHA plugin when using Recaptcha

The Recaptcha plugin class name does not follow the phpbb_captcha_ prefix convention as all others. Instead it simply uses phpbb_, hence the check for its existence under phpbb/captcha/plugins/ fails.

Depends on https://github.com/phpbb/phpbb/pull/3148

[PHPBB3-13342](https://tracker.phpbb.com/browse/PHPBB3-13342)
why not `$captcha_plugin === 'phpbb_recaptcha'`
we might match too much now?