Simplify capture pruning margin formula

Using just `PawnValueEg * depth` as Capture Prune Margin. There was a bunch
of patches that passed recently regarding captures, maybe this part of the
master code redundant? The patch was tested as a simplification:

STC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 20833 W: 4218 L: 4096 D: 12519
http://tests.stockfishchess.org/tests/view/5b1cf2100ebc5902ab9c6651

LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 27050 W: 3975 L: 3864 D: 19211
http://tests.stockfishchess.org/tests/view/5b1cfdc80ebc5902ab9c6776

Closes official-stockfish#1643

Bench: 4980482

src/search.cpp

@@ -71,16 +71,6 @@ namespace {
     return Value((175 - 50 * improving) * d / ONE_PLY);
   }
 
-  // Margin for pruning capturing moves: almost linear in depth
-  constexpr int CapturePruneMargin[] = { 0,
-                                         1 * PawnValueEg * 1055 / 1000,
-                                         2 * PawnValueEg * 1042 / 1000,
-                                         3 * PawnValueEg * 963  / 1000,
-                                         4 * PawnValueEg * 1038 / 1000,
-                                         5 * PawnValueEg * 950  / 1000,
-                                         6 * PawnValueEg * 930  / 1000
-                                       };
-
   // Futility and reductions lookup tables, initialized at startup
   int FutilityMoveCounts[2][16]; // [improving][depth]
   int Reductions[2][2][64][64];  // [pv][improving][depth][moveNumber]
@@ -963,7 +953,7 @@ namespace {
           }
           else if (    depth < 7 * ONE_PLY // (~20 Elo)
                    && !extension
-                   && !pos.see_ge(move, -Value(CapturePruneMargin[depth / ONE_PLY])))
+                   && !pos.see_ge(move, -Value(PawnValueEg * (depth / ONE_PLY))))
                   continue;
       }