devel/freebsd-gcc6: Retire obsolete port.

Supported FreeBSD versions can no longer be built with GCC 6 and
users wishing to compile FreeBSD with GCC are encouraged to use
devel/freebsd-gcc9 or devel/freebsd-gcc12 instead.

Reviewed by: manu, otis
Differential Revision: https://reviews.freebsd.org/D39218