kvm: x86: Cleanup vcpu->arch.guest_xstate_size

vcpu->arch.guest_xstate_size lost its only user since commit df1daba7
("KVM: x86: support XSAVES usage in the host"), so clean it up.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Message-Id: <20200429154312.1411-1-xiaoyao.li@intel.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>