Retiring WITHOUT_CXX

Several base system components are written in C++, and the WITHOUT_CXX
option is not regularly tested and is often broken. I fixed a number
of WITHOUT_CXX issues in response to Michael Dexter's recent Build
Option Survey runs, but it will break again absent ongoing effort.
This does not seem like a useful endeavour given the limitations it
imposes on the resulting system.

I'm proposing we remove the WITHOUT_CXX option and have opened a
review to do so: https://reviews.freebsd.org/D33108