mkosi: Drop CPUs= limit

mkosi: Drop CPUs= limit

Limiting VMs to 2 cpus was cargo culting without any
actual data that this benefits performance. The host OS
has a scheduler, let's make use of it and give the VM access
to all the CPUs. This doesn't mean they become inaccessible to
the host, it just means the VM gets as many virtual CPUs as the
host has CPU cores (threads). How they get scheduled is still up
to the host OS.