Simplify reduction on rootNode when bestMoveChanges is high
The reduction introduced in #3736 also consider on rootNode, so we don't have to reduce again.
STC:
LLR: 2.94 (-2.94,2.94) <-2.25,0.25>
Total: 28736 W: 7494 L: 7329 D: 13913
Ptnml(0-2): 95, 3247, 7503, 3444, 79
https://tests.stockfishchess.org/tests/view/61a3abe01b7fdf52228e74d8
LTC:
LLR: 2.94 (-2.94,2.94) <-2.25,0.25>
Total: 47816 W: 12434 L: 12308 D: 23074
Ptnml(0-2): 37, 4972, 13755, 5116, 28
https://tests.stockfishchess.org/tests/view/61a3c3e39f0c43dae1c71d71
closes https://github.com/official-stockfish/Stockfish/pull/3817
bench: 6331638