Simplify continuation history pruning

Non-regression STC:
LLR:  2.96/2.94<-6.00, 0.00> Elo diff: 6.13 [0.82, 11.43] (95%)
Games: 3800 W: 488 L: 421 D: 2891 Draw ratio: 76.1%
Pntl: [21, 313, 1173, 364, 29]

Non-regression LTC:
LLR:  2.94/2.94<-6.00, 0.00> Elo diff: 1.82 [-1.14, 4.78] (95%)
Games: 7639 W: 608 L: 568 D: 6463 Draw ratio: 84.6%
Pntl: [16, 475, 2790, 529, 9]

Bench: 2671284

src/Search.cpp

@@ -550,7 +550,7 @@ namespace Search
                     if (depth < 7 && n_moves > 3 + depth * depth)
                         continue;
 
-                    if (move_score < -100 * (depth - 1) - 50 * int(depth) * depth * depth)
+                    if (move_score < -600 * depth * depth)
                         continue;
 
                     if (!InCheck && depth < 12 && data.static_eval + 100 + 150 * depth + move_score / 75 < alpha)