Retire run-time detection of hardware POPCNT

It was meant to build a single binary optimized
for any kind of CPU: with and without hardware POPCNT.

This is a nice idea but in practice was never used, or
people builds binary with popcnt enabled or not, mainly
according to their type of CPU. And it was also never
used in the official Jim's builds where, in case, would
be easier for a number of reasons, do build two different
versions: with and without SEE42 support.

So retire this feature and simplify the code.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>