OptionalObsoleteFiles.inc: remove MK_CXX rule for usr/bin/c++

In fact MK_CXX does not control whether /usr/bin/c++ is built -- it is
installed as a link to Clang (which is always a C/C++ compiler), and it
already exists in OptionalObsoleteFiles under MK_TOOLCHAIN.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit c3f345ae3c0fac0684f83cff72ae23da18468777)