[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
[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created, thus the check is no longer valid and ends up resetting the value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.

PHPBB3-13315
[ticket/13315] Check for the includes/captcha/plugins/ directory instead.

PHPBB3-13315
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.