KVM: x86/mmu: Remove unused "kvm" of kvm_mmu_unlink_parents()

The "struct kvm *kvm" parameter of kvm_mmu_unlink_parents()
is not used, so remove it. No functional change intended.

Signed-off-by: Jinrong Liang <cloudliang@tencent.com>
Message-Id: <20220125095909.38122-2-cloudliang@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>