x86/virt: KVM: Move "disable SVM" helper into KVM SVM

Move cpu_svm_disable() into KVM proper now that all hardware
virtualization management is routed through KVM.  Remove the now-empty
virtext.h.

No functional change intended.

Reviewed-by: Kai Huang <kai.huang@intel.com>
Link: https://lore.kernel.org/r/20230721201859.2307736-17-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>