FROMGIT: ARM: Kill __smp_cross_call and co

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

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

Bug: 140053385
(cherry picked from commit 8aa837cb
 https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git

 irq/ipi-as-irq)
Change-Id: Ib1feecf3fc23c76277c50c872769f0ff2dd1bbd9
Signed-off-by: Eric Biggers <ebiggers@google.com>