Simplify futility equation

Simplify futility equation

This is a functional simplification. The 178 constant for the futility equation
in master can be removed.

STC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 42626 W: 9508 L: 9428 D: 23690
http://tests.stockfishchess.org/tests/view/5d5d4e320ebc5925cf11254e

LTC
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 26182 W: 4432 L: 4320 D: 17430
http://tests.stockfishchess.org/tests/view/5d5df70d0ebc5925cf112fee

Closes https://github.com/official-stockfish/Stockfish/pull/2278

Bench: 3985701