Stop installing /usr/bin/clang

Please look at https://reviews.freebsd.org/D21060
I propose to stop installing /usr/bin/clang, clang++, clang-cpp.

It probably does not matter when all your software comes from ports or
packages, but is actually very annoying when developing on FreeBSD.
In particular, you never know which `clang' is called in the user
environment, because it depends on the $PATH elements ordering.

To clear some confusion: this has nothing to do with not installing
compiler from base, /usr/bin/c{c,++,pp} are still there after the change
is applied.  It only to make clang on par with gcc, and to remove one
thing that was quite time-consuming in multi-target environment for me
during porting something large in FreeBSD userspace.