Simplification of partial_insertion_sort formula.
Passed STC:
https://tests.stockfishchess.org/tests/view/6590110879aa8af82b9562e9
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 134880 W: 34468 L: 34355 D: 66057
Ptnml(0-2): 476, 16060, 34220, 16243, 441
Passed LTC:
https://tests.stockfishchess.org/tests/view/659156ca79aa8af82b957f07
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 60780 W: 15179 L: 14996 D: 30605
Ptnml(0-2): 27, 6847, 16464, 7020, 32
closes https://github.com/official-stockfish/Stockfish/pull/4955
Bench: 1338331