coreos-init: remove default coreos-setup-environment

The coreos-setup-environment script can now only be provided by OEMs,
the default script that used to run when the OEM didn't provide one was
not smart enough to wait for networking to be configured before writing
out /etc/environment *but* we cannot simply add such a wait because that
would make it impossible for users to use cloud config in configdrive or
similar to configure static networking.

Longer term coreos-cloudinit will replace at least some of the current
OEM coreos-setup-environment scripts and perhaps eventually integrate
with networkd to cover the default case reasonably well. Sorting this
out is still ongoing.

(This ebuild bump was supposed to land a week ago but we missed it.)