Simplify cmh pruning

Simplify cmh pruning by removing PvNode exception

STC:
LLR: 2.96 (-2.94,2.94) [-3.00,1.00]
Total: 9935 W: 2330 L: 2184 D: 5421
http://tests.stockfishchess.org/tests/view/5b587dc00ebc5902bdb88424

LTC:
LLR: 2.95 (-2.94,2.94) [-3.00,1.00]
Total: 20635 W: 3585 L: 3464 D: 13586
http://tests.stockfishchess.org/tests/view/5b58910a0ebc5902bdb885b9

Closes official-stockfish#1711

Bench: 4905530

src/search.cpp

@@ -935,8 +935,7 @@ namespace {
  int lmrDepth = std::max(newDepth - reduction<PvNode>(improving, depth, moveCount), DEPTH_ZERO) / ONE_PLY;
 
  // Countermoves based pruning (~20 Elo)
- if ( lmrDepth < 4
- && (lmrDepth < 3 || ((ss - 1)->statScore > 0 && !PvNode))
+ if ( lmrDepth <= ((ss-1)->statScore > 0 ? 3 : 2)
  && (*contHist[0])[movedPiece][to_sq(move)] < CounterMovePruneThreshold
  && (*contHist[1])[movedPiece][to_sq(move)] < CounterMovePruneThreshold)
  continue;