Simplify move history reduction

Recent VLTC search tuning has suggested that the depth limit can be increased
by a lot. This patch simplifies away the depth-based bonus from statScore
reduction, making the divisor a constant.

src/search.cpp

@@ -1034,8 +1034,8 @@ Value search(Position& pos, Stack* ss, Value alpha, Value beta, Depth depth, boo
                       + (*contHist[1])[movedPiece][to_sq(move)]
                       + (*contHist[3])[movedPiece][to_sq(move)] - 4017;
 
-        // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
-        r -= ss->statScore / (11042 + 4510 * (depth > 8 && depth < 17));
+        // Decrease/increase reduction for moves with a good/bad history (~25 Elo)
+        r -= ss->statScore / 15694;
 
         // Step 16. Late moves reduction / extension (LMR, ~98 Elo)
         // We use various heuristics for the sons of a node after the first son has