updates playerCanMate (insufficient material) algorithm in Board

this is still incorrect - a proper implementation needs to check
the actual position, to account for locked pawn structures etc.

the old algorithm was probably a better practical compromise, so
this commit is mostly just for saving for review later.
Revert "updates playerCanMate (insufficient material) algorithm in Board"

This reverts commit 3ec58a169eab72b29cebbbe169fa49229f8e20ef.

see the commit message - the old version was better but both are
technically incorrect