tools: Drop cockpit-machines for Ubuntu 18.04
tools: Drop cockpit-machines for Ubuntu 18.04

Ubuntu 18.04 does not have libvirt-dbus package so cockpit-machines
was using virsh to communicate with libvirt on that distro version.
We stopped adding new features to the cockpit-machines code implemented
with virsh long time now.
Let's just stop updating the package there.

cockpituous/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,deryni/cockpit,garrett/cockpit,cockpituous/cockpit,mvollmer/cockpit,garrett/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,deryni/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,deryni/cockpit,martinpitt/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,deryni/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,cockpit-project/cockpit,deryni/cockpit,garrett/cockpit,moraleslazaro/cockpit,deryni/cockpit,cockpituous/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,cockpituous/cockpit,garrett/cockpit,deryni/cockpit,martinpitt/cockpit