sparc:remove unused declaration smp_boot_cpus()

smp_boot_cpus() was replaced smp_prepare_cpus() long ago, and it no
longer needed, so delete it.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>