KVM: VMX: Remove defunct "nr_active_uret_msrs" field

Remove vcpu_vmx.nr_active_uret_msrs and its associated comment, which are
both defunct now that KVM keeps the list constant and instead explicitly
tracks which entries need to be loaded into hardware.

No functional change intended.

Fixes: ee9d22e0

 ("KVM: VMX: Use flag to indicate "active" uret MSRs instead of sorting list")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210908002401.1947049-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>