KVM: Drop largepages_enabled and its accessor/mutator

Drop largepages_enabled, kvm_largepages_enabled() and
kvm_disable_largepages() now that all users are gone.

Note, largepages_enabled was an x86-only flag that got left in common
KVM code when KVM gained support for multiple architectures.

No functional change intended.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>