Remove Reduce Depth

Remove Reduce Depth at PV nodes.

STC:
LLR: 2.94 (-2.94,2.94) {-1.25,0.25}
Total: 56760 W: 6299 L: 6236 D: 44225
Ptnml(0-2): 286, 4843, 18076, 4872, 303
https://tests.stockfishchess.org/tests/view/5f41356087a5c3c63d8f53c9

LTC:
LLR: 2.95 (-2.94,2.94) {-0.75,0.25}
Total: 17496 W: 954 L: 865 D: 15677
Ptnml(0-2): 13, 768, 7098, 855, 14
https://tests.stockfishchess.org/tests/view/5f41bb7687a5c3c63d8f53f9

closes https://github.com/official-stockfish/Stockfish/pull/3055

Bench: 3555051

src/search.cpp

@@ -940,12 +940,6 @@ namespace {
             }
     }
 
-    // Step 11. If the position is not in TT, decrease depth by 2
-    if (   PvNode
-        && depth >= 6
-        && !ttMove)
-        depth -= 2;
-
 moves_loop: // When in check, search starts from here
 
     const PieceToHistory* contHist[] = { (ss-1)->continuationHistory, (ss-2)->continuationHistory,
@@ -969,7 +963,7 @@ moves_loop: // When in check, search starts from here
     // Mark this node as being searched
     ThreadHolding th(thisThread, posKey, ss->ply);
 
-    // Step 12. Loop through all pseudo-legal moves until no moves remain
+    // Step 11. Loop through all pseudo-legal moves until no moves remain
     // or a beta cutoff occurs.
     while ((move = mp.next_move(moveCountPruning)) != MOVE_NONE)
     {
@@ -1007,7 +1001,7 @@ moves_loop: // When in check, search starts from here
       // Calculate new depth for this move
       newDepth = depth - 1;
 
-      // Step 13. Pruning at shallow depth (~200 Elo)
+      // Step 12. Pruning at shallow depth (~200 Elo)
       if (  !rootNode
           && pos.non_pawn_material(us)
           && bestValue > VALUE_TB_LOSS_IN_MAX_PLY)
@@ -1065,7 +1059,7 @@ moves_loop: // When in check, search starts from here
           }
       }
 
-      // Step 14. Extensions (~75 Elo)
+      // Step 13. Extensions (~75 Elo)
 
       // Singular extension search (~70 Elo). If all moves but one fail low on a
       // search of (alpha-s, beta-s), and just one fails high on (alpha, beta),
@@ -1148,10 +1142,10 @@ moves_loop: // When in check, search starts from here
                                                                 [movedPiece]
                                                                 [to_sq(move)];
 
-      // Step 15. Make the move
+      // Step 14. Make the move
       pos.do_move(move, st, givesCheck);
 
-      // Step 16. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
+      // Step 15. Reduced depth search (LMR, ~200 Elo). If the move fails high it will be
       // re-searched at full depth.
       if (    depth >= 3
           &&  moveCount > 1 + 2 * rootNode + 2 * (PvNode && abs(bestValue) < 2)
@@ -1254,7 +1248,7 @@ moves_loop: // When in check, search starts from here
           didLMR = false;
       }
 
-      // Step 17. Full depth search when LMR is skipped or fails high
+      // Step 16. Full depth search when LMR is skipped or fails high
       if (doFullDepthSearch)
       {
           value = -search<NonPV>(pos, ss+1, -(alpha+1), -alpha, newDepth, !cutNode);
@@ -1282,12 +1276,12 @@ moves_loop: // When in check, search starts from here
           value = -search<PV>(pos, ss+1, -beta, -alpha, newDepth, false);
       }
 
-      // Step 18. Undo move
+      // Step 17. Undo move
       pos.undo_move(move);
 
       assert(value > -VALUE_INFINITE && value < VALUE_INFINITE);
 
-      // Step 19. Check for a new best move
+      // Step 18. Check for a new best move
       // Finished searching the move. If a stop occurred, the return value of
       // the search cannot be trusted, and we return immediately without
       // updating best move, PV and TT.
@@ -1364,7 +1358,7 @@ moves_loop: // When in check, search starts from here
         return VALUE_DRAW;
     */
 
-    // Step 20. Check for mate and stalemate
+    // Step 19. Check for mate and stalemate
     // All legal moves have been searched and if there are no legal moves, it
     // must be a mate or a stalemate. If we are in a singular extension search then
     // return a fail low score.