Insufficient

Remove check for insufficient mating material from Position::is_draw().
Kicks only in rather rare (0.00114% in bench 17), and draw evaluation for KK, KNK and KBK is handled by material.cpp.

Passed LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 33023 W: 4613 L: 4509 D: 23901

and STC, but then test was purged.
STC after 128k games: (after test was purged)
LLR: 0.05 (-2.94,2.94) [-3.00,1.00]
Total: 128000 W: 21357 L: 21560 D: 85083