Revert "pc: Enforce adding CPUs contiguously and removing them in opposite order"

This reverts commit 4da7faaeb0c7dd3f7f233165d336c878f78fd1eb.

Since commit:
  pc: init CPUState->cpu_index with index in possible_cpus[]
cpu_index is stable regardless of the order cpus were created
and QEMU instance stays migratable always so limitation added
by 4da7faaeb could be safely removed.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
