bsd.port.mk: remove unused base system binutils support

GNU binutils 2.17.50 was removed from the base system (for all archs)
prior to FreeBSD 13.0. Remove ports support for base system ld.bfd.

Reviewed by: portmgr (mat)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46015