Remove combineLast3 optimization
Passed non-reg STC 1st:
LLR: 2.96 (-2.94,2.94) <-1.75,0.25>
Total: 67328 W: 17296 L: 17118 D: 32914
Ptnml(0-2): 158, 7095, 19011, 7211, 189
https://tests.stockfishchess.org/tests/view/67e6c2796682f97da2178ebe
Passed non-reg STC 2nd:
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 92288 W: 23885 L: 23734 D: 44669
Ptnml(0-2): 213, 10039, 25518, 10132, 242
https://tests.stockfishchess.org/tests/view/67ed6a2d31d7cf8afdc45190
closes https://github.com/official-stockfish/Stockfish/pull/5975
Bench: 1875196