clean(toolchain): Remove overlay toolchain, moved to portage-stable

We have to actually remove these as opposed to ~keywording them because
crossdev will always use packages from overlays if they exist.

This won't actually impact anyone's SDK just yet as update_chroot will
only use binary packages for the toolchains. It will keep checking for
updates but not do anything until those updates are available from the
binhost. So the next release will switch to the new toolchain.

Depends on https://github.com/coreos/portage-stable/pull/15