Revert "Mix alpha and statScore for reduction"

This reverts commit 8bab097.

In this form the patch reduces mate finding effectiveness, as the large alpha value has negative influence on the reductions.

see also #4183

Bench: 4114228

src/search.cpp

@@ -787,9 +787,9 @@ namespace {
     // The depth condition is important for mate finding.
     if (   !ss->ttPv
         &&  depth < 8
-        &&  eval - futility_margin(depth, improving) - (ss-1)->statScore / 301 >= beta
+        &&  eval - futility_margin(depth, improving) - (ss-1)->statScore / 303 >= beta
         &&  eval >= beta
-        &&  eval < 28692) // larger than VALUE_KNOWN_WIN, but smaller than TB wins
+        &&  eval < 28031) // larger than VALUE_KNOWN_WIN, but smaller than TB wins
         return eval;
 
     // Step 9. Null move search with verification search (~22 Elo)
@@ -1179,7 +1179,7 @@ namespace {
                          - 4433;
 
           // Decrease/increase reduction for moves with a good/bad history (~30 Elo)
-          r -= (ss->statScore + 5 * alpha) / 15448;
+          r -= ss->statScore / 13628;
 
           // In general we want to cap the LMR depth search at newDepth, but when
           // reduction is negative, we allow this move a limited search extension