Always check repetition

It seems stronger both at fast 15+0.05 TC with fixed game number test:
ELO: 2.74 +-2.7 (95%) LOS: 97.6%
Total: 24000 W: 4698 L: 4509 D: 14793

And also at long 60+0.05 TC with SPRT
LLR: 3.05 (-2.94,2.94)
Total: 38986 W: 6845 L: 6547 D: 25594

bench: 5157061
De-templetize Position::is_draw()

Now that we always check for repetition we don't
need a template anymore.

No functional change.
Merge remote-tracking branch 'refs/remotes/MainFish/master' into master-2