Revert "pc: Add 'etc/boot-cpus' fw_cfg file for machine with more than 255 CPUs"

This reverts commit 080ac219.

Legacy FW_CFG_NB_CPUS will be reused instead of 'etc/boot-cpus'
fw_cfg file since it does the same and there is no point
to maintaing duplicate guest ABI, if it can be helped.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1479212236-183810-2-git-send-email-imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>