devel/freebsd-gcc9: Remove old GCC system toolchain

FreeBSD 13.0 and later can be compiled with devel/freebsd-gcc12
or devel/freebsd-gcc13 and GCC 9 is unsupported upstream.

PR: 269659
Reviewed by: diizzy, brooks, emaste
Differential Revision: https://reviews.freebsd.org/D43379