Simplify SMP Reduction

Passed Non-regression SMP STC:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 232784 W: 59845 L: 59841 D: 113098
Ptnml(0-2): 289, 26459, 62934, 26379, 331
https://tests.stockfishchess.org/tests/view/68ab96bf75da51a345a5acd6

Passed Non-regression SMP LTC:
LLR: 2.95 (-2.94,2.94) <-1.75,0.25>
Total: 374270 W: 95978 L: 96113 D: 182179
Ptnml(0-2): 118, 38575, 109888, 38432, 122
https://tests.stockfishchess.org/tests/view/68abcf1c75da51a345a5adb6

closes https://github.com/official-stockfish/Stockfish/pull/6285

Bench: 2667107

src/search.cpp

@@ -1167,8 +1167,7 @@ moves_loop:  // When in check, search starts here
 
         // These reduction adjustments have no proven non-linear scaling
 
-        r += 671;  // Base reduction offset to compensate for other tweaks
-        r -= (threadIdx % 8) * 64;
+        r += 543;  // Base reduction offset to compensate for other tweaks
         r -= moveCount * 66;
         r -= std::abs(correctionValue) / 30450;