Retire WITHOUT_CXX option

Several important base system components are written in C++, and the
WITHOUT_CXX option produced a system that was not fully functional.
Just accept this, and remove the option to build without C++ support.

This reverts commit adc3c128c6603054586a993d117e5dd808deac17.

Reviewed by: brooks, kevans, jhb (earlier)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33108