Never clear stats

Based on an idea and patch by VoyagerOne.

Small simplification, but was tedted for an ELO gain anyway.

STC:
LLR: 2.95 (-2.94,2.94) [-1.00,4.00]
Total: 5375 W: 1119 L: 977 D: 3279

LTC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 17893 W: 2984 L: 2792 D: 12117

bench 8322847

src/search.cpp

@@ -339,10 +339,6 @@ namespace {
     beta = VALUE_INFINITE;
 
     TT.new_search();
-    History.clear();
-    CounterMovesHistory.clear();
-    Gains.clear();
-    Countermoves.clear();
 
     size_t multiPV = Options["MultiPV"];
     Skill skill(Options["Skill Level"]);