ARM: shmobile: apmu: Remove obsolete shmobile_smp_apmu_prepare_cpus()

After the removal of the legacy SMP fallbacks, there are no more users
left of shmobile_smp_apmu_prepare_cpus().

Remove it, together with the legacy SMP config parser.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>