Simplify internal iterative reductions
This is a revert of cc992e5.
This patch is based on consistent observations that decreasing depth more in IIR generally has a bad scaling behaviour (good at STC, bad at longer time controls).
Simplification STC: https://tests.stockfishchess.org/tests/view/6689266659cb3228a4759b26
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 96992 W: 24977 L: 24824 D: 47191
Ptnml(0-2): 251, 11497, 24851, 11642, 255
Simplification LTC: https://tests.stockfishchess.org/tests/view/668930ffe59d990b103f6ab1
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 35808 W: 9185 L: 8980 D: 17643
Ptnml(0-2): 25, 3776, 10101, 3973, 29
closes https://github.com/official-stockfish/Stockfish/pull/5447
Bench: 1097766