Tweak history initialization

Simplify initialization of continuation history by using everywhere the same starting value.

STC:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 90952 W: 24312 L: 24153 D: 42487
Ptnml(0-2): 356, 10168, 24290, 10285, 377
https://tests.stockfishchess.org/tests/view/633948f235f43d649ff61fd0

LTC:
LLR: 2.96 (-2.94,2.94) <-1.75,0.25>
Total: 162416 W: 43540 L: 43466 D: 75410
Ptnml(0-2): 77, 16289, 48417, 16333, 92
https://tests.stockfishchess.org/tests/view/6339ee8a35f43d649ff63986

closes  https://github.com/official-stockfish/Stockfish/pull/4186

Bench: 4156027

src/search.h

@@ -31,9 +31,6 @@ class Position;
 
 namespace Search {
 
-/// Threshold used for countermoves based pruning
-constexpr int CounterMovePruneThreshold = 0;
-
 
 /// Stack struct keeps track of the information we need to remember from nodes
 /// shallower and deeper in the tree during the search. Each search thread has