x86/kvm/vmx: remove unused variable in segment_base()

The pointer 'struct desc_struct *d' is unused since commit 8c2e41f7


("x86/kvm/vmx: Simplify segment_base()") so let's remove it.

Signed-off-by: Jérémy Lefaure <jeremy.lefaure@lse.epita.fr>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>