x86/kvm: remove unused ack_notifier callbacks

Commit f5244726 ("KVM: irq ack notification") introduced an
ack_notifier() callback in struct kvm_pic and in struct kvm_ioapic
without using them anywhere. Remove those callbacks again.

Signed-off-by: Juergen Gross <jgross@suse.com>
Message-Id: <20211117071617.19504-1-jgross@suse.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>