Simplify FullThreats::append_active_indices()
Also add an else to FullThreats::append_changed_indices() because dp2removed
can't be equal to both from and to simultaneously.
passed STC
https://tests.stockfishchess.org/tests/view/69e71a490e9667dd5a76549f
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 210688 W: 54362 L: 54335 D: 101991
Ptnml(0-2): 613, 23246, 57593, 23285, 607
Thanks to ces42 for an improvement suggestion.
closes https://github.com/official-stockfish/Stockfish/pull/6770
No functional change