Remove support for host_tools

This was envisioned for downloading supplemental tooling beyond what was
kept in the toolchains package. You could imagine this being useful for
distributing mksquashfs, fwup, and any number of other host tools used
by Nerves. Distributing them separate from toolchains was highly
desirable to avoid coupling releases.

Sadly this was never used. Deleting the code removes the need to
maintain support.

Author: fhunleth