Simplify NMP condition

As this is tried only for cutNode, and all children of cutNodes will be nonCutNodes, the guard condition is redundant.
Simplification just removes the unnecessary condition (always true).

closes https://github.com/official-stockfish/Stockfish/pull/6187

No functional change

AUTHORS

@@ -231,6 +231,7 @@ Stefano Di Martino (StefanoD)
 Steinar Gunderson (sesse)
 Stéphane Nicolet (snicolet)
 Stephen Touset (stouset)
+Styx (styxdoto)
 Syine Mineta (MinetaS)
 Taras Vuk (TarasVuk)
 Thanar2

src/search.cpp

@@ -856,10 +856,11 @@ Value Search::Worker::search(
     }
 
     // Step 9. Null move search with verification search
-    if (cutNode && (ss - 1)->currentMove != Move::null()
-        && ss->staticEval >= beta - 19 * depth + 403 && !excludedMove && pos.non_pawn_material(us)
-        && ss->ply >= nmpMinPly && !is_loss(beta))
+    if (cutNode && ss->staticEval >= beta - 19 * depth + 403 && !excludedMove
+        && pos.non_pawn_material(us) && ss->ply >= nmpMinPly && !is_loss(beta))
     {
+        assert((ss - 1)->currentMove != Move::null());
+
         // Null move dynamic reduction based on depth
         Depth R = 7 + depth / 3;