Remove unused code from the post NNUE accumulator merge

After merging the HalfKA and Threats accumulators (7c7fe322) and the
subsequent removal of the double-incremental/fused update, a number of
NNUE helpers and fields became unreachable. Each was verified to have
zero callers/readers across the source tree:

- FusedUpdateData logic in FullThreats: the fused-update branch of
  append_changed_indices and the FusedUpdateData parameter are unused;
  the accumulator update no longer passes fused data.
- FullThreats::requires_refresh: never called. The live king-bucket
  refresh check is HalfKAv2_hm::requires_refresh (PSQFeatureSet), used
  in nnue_accumulator.
- HalfKAv2_hm::append_active_indices: never called. The live
  active-index builder is FullThreats::append_active_indices
  (ThreatFeatureSet).
- DirtyThreats::us, prevKsq and ksq: written in do_move but only read by
  the now-removed FullThreats::requires_refresh. Removing them also
  drops three stores from the do_move path.
- Unused feature Name constants and the unused FtOneVal / HiddenMaxVal
  constants in nnue_common.h.
- Two stale feature-header banner comments.

closes https://github.com/official-stockfish/Stockfish/pull/6898

No functional change

src/types.h

@@ -339,8 +339,6 @@ using DirtyThreatList = ValueList<DirtyThreat, 96>;
 
 struct DirtyThreats {
     DirtyThreatList list;
-    Color           us;
-    Square          prevKsq, ksq;
 };
 
     #define ENABLE_INCR_OPERATORS_ON(T) \