Remove captcha.enabled from xwiki.properties file because the setting is superseded.

The captcha module includes a setting which is not used because the two uses of the captcha module are registration and commenting which both have captcha enable settings.
see: http://lists.xwiki.org/pipermail/devs/2010-March/017736.html

Plan: Remove functionality for v2.3 but leave and deprecate isEnabled() (which will always return true) until 2.4 when that too will be removed.