ARM: shmobile: r8a7779: Remove redundant cpu_disable implementation

By default only the non-boot CPUs can be hotplugged if the
smp_operations structure doesn't have the cpu_disable function
pointer set. r8a7779_cpu_disable() implements the same logic,
only non-boot CPUs can be hotplugged, so it's redundant to
override the default behavior.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>