speed up update_piece_threats after SFNNv12
with the removal of all king-related threats in SFNNv12 we can avoid some work in update_piece_threats
STC for anematode's swan:
https://tests.stockfishchess.org/tests/view/6986b920b0f3ca5200aaf80a
LLR: 2.95 (-2.94,2.94) <0.00,2.00>
Total: 262880 W: 68379 L: 67747 D: 126754
Ptnml(0-2): 808, 29001, 71223, 29567, 841
STC of further speedups on top of swan:
https://tests.stockfishchess.org/tests/view/698d7f8362a8f472da3a8a35
LLR: 2.93 (-2.94,2.94) <0.00,2.00>
Total: 65920 W: 17126 L: 16781 D: 32013
Ptnml(0-2): 186, 7084, 18105, 7369, 216
closes https://github.com/official-stockfish/Stockfish/pull/6614
No functional change
Co-Authored-By: Timothy Herchen <timothy.herchen@gmail.com>