Simplify futility pruning return value

Return eval as it is while doing futility pruning.

STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 167687 W: 29778 L: 29904 D: 108005

LTC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 26905 W: 3503 L: 3390 D: 20012

Bench: 5936728

src/search.cpp

@@ -743,7 +743,7 @@ namespace {
         &&  eval - futility_margin(depth) >= beta
         &&  eval < VALUE_KNOWN_WIN  // Do not return unproven wins
         &&  pos.non_pawn_material(pos.side_to_move()))
-        return eval - futility_margin(depth);
+        return eval;
 
     // Step 8. Null move search with verification search (is omitted in PV nodes)
     if (   !PvNode