Make razor margin depth independent

STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 37171 W: 6680 L: 6587 D: 23904

LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 11632 W: 1574 L: 1442 D: 8616

bench: 5098576

src/search.cpp

@@ -67,8 +67,7 @@ namespace {
   const int skipPhase[] = { 0, 1, 0, 1, 2, 3, 0, 1, 2, 3, 4, 5, 0, 1, 2, 3, 4, 5, 6, 7 };
 
   // Razoring and futility margin based on depth
-  // razor_margin[0] is unused as long as depth >= ONE_PLY in search
-  const int razor_margin[] = { 0, 570, 603, 554 };
+  const int razor_margin = 600;
   Value futility_margin(Depth d) { return Value(150 * d / ONE_PLY); }
 
   // Futility and reductions lookup tables, initialized at startup
@@ -654,12 +653,12 @@ namespace {
     // Step 6. Razoring (skipped when in check)
     if (   !PvNode
         &&  depth < 4 * ONE_PLY
-        &&  eval + razor_margin[depth / ONE_PLY] <= alpha)
+        &&  eval + razor_margin <= alpha)
     {
         if (depth <= ONE_PLY)
             return qsearch<NonPV, false>(pos, ss, alpha, alpha+1);
 
-        Value ralpha = alpha - razor_margin[depth / ONE_PLY];
+        Value ralpha = alpha - razor_margin;
         Value v = qsearch<NonPV, false>(pos, ss, ralpha, ralpha+1);
         if (v <= ralpha)
             return v;