Remove prefetching in Position::do_null_move
Passed non-regression STC, Hash=16:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 60704 W: 15709 L: 15522 D: 29473
Ptnml(0-2): 169, 6589, 16649, 6776, 169
https://tests.stockfishchess.org/tests/view/6963d381f6d2e8deff76918f
Passed non-regression STC SMP, Hash=512:
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 313512 W: 80151 L: 80231 D: 153130
Ptnml(0-2): 376, 35492, 85111, 35390, 387
https://tests.stockfishchess.org/tests/view/696e1684e5a70879bd3fe38f
closes https://github.com/official-stockfish/Stockfish/pull/6545
No functional change