simplify FullThreats::append_active_indices
also requires de-templating shift
passed STC: https://tests.stockfishchess.org/tests/view/6a78a8c1a7e815d01b5601c4
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 67904 W: 17662 L: 17490 D: 32752
Ptnml(0-2): 135, 7037, 19416, 7249, 115
A benchmark also shows no regression:
```
Result of 100 runs (counting nodes/1B cycles)
=============================================
base (...tockfish.5062ae) = 275495 +/- 3270
test (...tockfish.978b83) = 275951 +/- 3308
speedup % = +0.16 +/- 0.07
[95% CI; t-test]
likelihood(speedup > 0) = 1.0000
CPU: 16 x Intel(R) Core(TM) Ultra 9 185H
```
closes https://github.com/official-stockfish/Stockfish/pull/7047
No functional change