Remove support for auto-selecting an external binutils.

All of the in-tree architectures not supported by in-tree binutils are
supported by lld, so the condition is now always false. It also
didn't fully work since the external binutils are installed into a
directory that uses the host's OS version, not the target OS version.

Reviewed by: emaste, imp
Differential Revision: https://reviews.freebsd.org/D23294