Simplify Away Countermove Heuristic

Passed Non-regression STC:
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 977824 W: 252072 L: 252888 D: 472864
Ptnml(0-2): 2518, 117120, 250560, 116088, 2626
https://tests.stockfishchess.org/tests/view/6683452d95b0d1e881e81541

Passed Non-regression LTC:
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 81048 W: 20630 L: 20470 D: 39948
Ptnml(0-2): 36, 8915, 22464, 9071, 38
https://tests.stockfishchess.org/tests/view/66886b7b0c9d7c1ab33ed281

closes https://github.com/official-stockfish/Stockfish/pull/5441

bench 1276784

src/movepick.cpp

@@ -91,7 +91,6 @@ MovePicker::MovePicker(const Position&              p,
                        const CapturePieceToHistory* cph,
                        const PieceToHistory**       ch,
                        const PawnHistory*           ph,
-                       Move                         cm,
                        const Move*                  killers) :
     pos(p),
     mainHistory(mh),
@@ -99,7 +98,7 @@ MovePicker::MovePicker(const Position&              p,
     continuationHistory(ch),
     pawnHistory(ph),
     ttMove(ttm),
-    refutations{{killers[0], 0}, {killers[1], 0}, {cm, 0}},
+    refutations{{killers[0], 0}, {killers[1], 0}},
     depth(d) {
     assert(d > 0);
 
@@ -273,10 +272,6 @@ top:
         cur      = std::begin(refutations);
         endMoves = std::end(refutations);
 
-        // If the countermove is the same as a killer, skip it
-        if (refutations[0] == refutations[2] || refutations[1] == refutations[2])
-            --endMoves;
-
         ++stage;
         [[fallthrough]];