arm64: Kill __smp_cross_call and co

The old IPI registration interface is now unused on arm64, so let's
get rid of it.

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>