Remove sorting optimization for many zeroes

Remove sorting optimization for many zeroes

With negative history we don't have anymore a
lot of zeroes to score, so just split moves in
positives and non-positives sets.

Speed up is almost zero, we cannot test speed directly
because node count changed due to reorder, but I have
verified sorting is correct. With a profiler I have
seen we gain a little in sort_moves() and lose a little
in insertion_sort(), so the net effect is almost zero,
but code is simpler.

No real change, just move reordering.

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