Simplify condition for ProbCut move count pruning
We retire depth-dependence in ProbCut move count pruning,
the move count limit condition is now : "probCutCount < 3"
STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 33895 W: 6995 L: 6897 D: 20003
http://tests.stockfishchess.org/tests/view/5aa6eaba0ebc59029781009d
LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 28959 W: 4441 L: 4333 D: 20185
http://tests.stockfishchess.org/tests/view/5aa73dfa0ebc5902978100be
Ideas for future work:
• Is a flat move count limit in ProbCut ideal? Depth dependence, or
dependence on some other variable, could possibly be reintroduced.
• The move count limit 3 is untuned and a better value may exist.
Closes https://github.com/official-stockfish/Stockfish/pull/1486
Bench: 5741807