KVM: PPC: Remove unused argument to kvmppc_core_dequeue_external

Currently kvmppc_core_dequeue_external() takes a struct kvm_interrupt *
argument and does nothing with it, in any of its implementations.
This removes it in order to make things easier for forthcoming
in-kernel interrupt controller emulation code.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>