KVM: nVMX: remove unnecessary #ifdef

nested_vmx_check_controls() has already run by the time KVM checks host state,
so the "host address space size" exit control can only be set on x86-64 hosts.
Simplify the condition at the cost of adding some dead code to 32-bit kernels.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>