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/position.cpp

@@ -862,12 +862,10 @@ void Position::do_move(Move                      m,
     Piece  pc       = piece_on(from);
     Piece  captured = m.type_of() == EN_PASSANT ? make_piece(them, PAWN) : piece_on(to);
 
-    dp.pc       = pc;
-    dp.from     = from;
-    dp.to       = to;
-    dp.add_sq   = SQ_NONE;
-    dts.us      = us;
-    dts.prevKsq = square<KING>(us);
+    dp.pc     = pc;
+    dp.from   = from;
+    dp.to     = to;
+    dp.add_sq = SQ_NONE;
 
     assert(color_of(pc) == us);
     assert(captured == NO_PIECE || color_of(captured) == (m.type_of() != CASTLING ? them : us));
@@ -1080,8 +1078,6 @@ void Position::do_move(Move                      m,
         }
     }
 
-    dts.ksq = square<KING>(us);
-
     assert(pos_is_ok());
 
     assert(dp.pc != NO_PIECE);