Simplify smallnet threshold
Turns the quadratic threshold to a linear one
STC non-reg:
https://tests.stockfishchess.org/tests/view/665ba0b744e8416a9cdc188d
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 330432 W: 85351 L: 85454 D: 159627
Ptnml(0-2): 888, 39643, 84283, 39488, 914
LTC non-reg:
https://tests.stockfishchess.org/tests/view/665cd60ffd45fb0f907c4306
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 139146 W: 35194 L: 35093 D: 68859
Ptnml(0-2): 58, 15523, 38313, 15618, 61
closes https://github.com/official-stockfish/Stockfish/pull/5342
Bench: 1057383