Retire asymmThreshold

Verified with 40K games at long TC does not regress:
ELO: 1.74 +-1.9 (95%) LOS: 96.2%
Total: 39624 W: 6402 L: 6203 D: 27019

bench: 7762310
Remove asymmThreshold stale comment

No functional change.
Merge remote-tracking branch 'upstream/master'