xen/arm: smpboot: drop unneeded code in start_secondary

xen/arm: smpboot: drop unneeded code in start_secondary

CPU0 boots up secondary CPUs one by one. Before booting
one secondary CPU, CPU0 will assign hwid to smp_up_cpu
and flush cache. After a secondary CPU boots up,
CPU0 will assign MPIDR_INVALID to smp_up_cpu and flush
cache.

There is no need for secondary CPUs to assign MPIDR_INVALID
to smp_up_cpu. So, drop it.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>