Reviewer Ellie - Remove onigumura from libevhtp

This removes the dependency on onigumura from libevhtp, a second change (coming up!) is needed in the parent projects (homestead/sprout) to pass `-DEVHTP_DISABLE_REGEX=YES` to `cmake` so we don't compile the onigumura code on each build.

I've built sprout and homestead with this change and run both of their UT frameworks.