KVM: SVM: remove unused field from struct vcpu_svm

KVM: SVM: remove unused field from struct vcpu_svm

The pointer to svm_cpu_data in struct vcpu_svm looks interesting from
the point of view of accessing it after vmexit, when the GSBASE is still
containing the guest value.  However, despite existing since the very
first commit of drivers/kvm/svm.c (commit b4d1853ff20b, "[PATCH] kvm:
userspace interface", 2006-12-10), it was never set to anything.

Ignore the opportunity to fix a 16 year old "bug" and delete it; doing
things the "harder" way makes it possible to remove more old cruft.

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