Set reduction to 0 if the move is a TT move

The reduction formula currently decreases by 1 if the move is a TT move.
This changes this by just setting the reduction to 0 instead.

Passed STC:
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 83136 W: 21145 L: 20758 D: 41233
Ptnml(0-2): 279, 9772, 21090, 10137, 290
https://tests.stockfishchess.org/tests/view/653c0fbacc309ae839561584

Passed LTC:
LLR: 2.96 (-2.94,2.94) <0.50,2.50>
Total: 273150 W: 67987 L: 67171 D: 137992
Ptnml(0-2): 155, 30730, 73966, 31592, 132
https://tests.stockfishchess.org/tests/view/653d9d02cc309ae839562fdf

closes https://github.com/official-stockfish/Stockfish/pull/4863

bench: 1110556

src/search.cpp

@@ -1147,9 +1147,10 @@ moves_loop:  // When in check, search starts here
         if ((ss + 1)->cutoffCnt > 3)
             r++;
 
-        // Decrease reduction for first generated move (ttMove)
+        // Set reduction to 0 for first generated move (ttMove)
+        // Nullifies all previous reduction adjustments to ttMove and leaves only history to do them
         else if (move == ttMove)
-            r--;
+            r = 0;
 
         ss->statScore = 2 * thisThread->mainHistory[us][from_to(move)]
                       + (*contHist[0])[movedPiece][to_sq(move)]