x86: cpu_init(): remove ugly #ifdef construct around debug register clear

Impact: Cleanup

While I was looking through the new and improved bootstrap code - great
work that, thanks! I found the below a slight improvement.

Remove unnecessary ugly #ifdef construct around debug register clear.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>