[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.

PHPBB3-13342
Merge pull request #3149 from prototech/ticket/13342

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

* prototech/ticket/13342:
  [ticket/13342] Fix invalid resetting of CAPTCHA plugin when using Recaptcha.