Simplify Null Move Search Reduction

slightly simpler formula for reduction computation.

first round of tests:
STC:
LLR: 2.97 (-2.94,2.94) <-2.50,0.50>
Total: 15632 W: 1319 L: 1204 D: 13109
Ptnml(0-2): 33, 956, 5733, 1051, 43
https://tests.stockfishchess.org/tests/view/60bd03c7457376eb8bcaa600

LTC:
LLR: 3.37 (-2.94,2.94) <-2.50,0.50>
Total: 86296 W: 2814 L: 2779 D: 80703
Ptnml(0-2): 33, 2500, 38039, 2551, 25
https://tests.stockfishchess.org/tests/view/60bd1ff0457376eb8bcaa653

recent tests:
STC:
LLR: 2.93 (-2.94,2.94) <-2.50,0.50>
Total: 23936 W: 1895 L: 1793 D: 20248
Ptnml(0-2): 40, 1470, 8869, 1526, 63
https://tests.stockfishchess.org/tests/view/611f9b7d4977aa1525c9cb6b

LTC:
LLR: 2.95 (-2.94,2.94) <-2.50,0.50>
Total: 62568 W: 1750 L: 1713 D: 59105
Ptnml(0-2): 19, 1560, 28085, 1605, 15
https://tests.stockfishchess.org/tests/view/611fa4814977aa1525c9cb71

functional on high depth

closes official-stockfish#3535

Bench: 5375286

src/search.cpp

@@ -801,7 +801,7 @@ namespace {
         assert(eval - beta >= 0);
 
         // Null move dynamic reduction based on depth and value
-        Depth R = (1090 + 81 * depth) / 256 + std::min(int(eval - beta) / 205, 3);
+        Depth R = std::min(int(eval - beta) / 205, 3) + depth / 3 + 4;
 
         ss->currentMove = MOVE_NULL;
         ss->continuationHistory = &thisThread->continuationHistory[0][0][NO_PIECE][0];