KVM: x86: drop unneeded null test

If the null test is needed, the call to cancel_delayed_work_sync would have
already crashed.  Normally, the destroy function should only be called
if the init function has succeeded, in which case ioapic is not null.

Problem found using Coccinelle.

Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>