Simplify futility pruning parent node
only continuation histories seem needed for this purpose.
STC:
http://tests.stockfishchess.org/tests/view/5e6b88dfe42a5c3b3ca2e4ab
LLR: 2.94 (-2.94,2.94) {-1.50,0.50}
Total: 113356 W: 21725 L: 21696 D: 69935
Ptnml(0-2): 1999, 13255, 26163, 13240, 2021
LTC:
http://tests.stockfishchess.org/tests/view/5e6babbfe42a5c3b3ca2e4c2
LLR: 2.94 (-2.94,2.94) {-1.50,0.50}
Total: 22164 W: 2917 L: 2821 D: 16426
Ptnml(0-2): 173, 2040, 6548, 2160, 161
closes https://github.com/official-stockfish/Stockfish/pull/2583
bench: 4839496