Remove support for auto-selecting an external binutils.

[jhb]

Summary

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.

Test Plan

• make tinderbox

[imp]

Glad to see we can get rid of this. Thanks for cleaning it up.

[emaste]

I've updated my local version of D23124 with this change