KVM: SVM: remove dead field from struct svm_cpu_data

The "cpu" field of struct svm_cpu_data has been write-only since commit
4b656b12 ("KVM: SVM: force new asid on vcpu migration", 2009-08-05).
Remove it.

Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>