Remove an useless assignment

The assignment (ss + 1)->excludedMove = Move::none() can be simplified away because when that line is reached, (ss + 1)->excludedMove is always already none. The only moment stack[x]->excludedMove is modified, is during singular search, but it is reset to none right after the singular search is finished.

closes #5153

No functional change

src/search.cpp

@@ -585,7 +585,7 @@ Value Search::Worker::search(
 
     assert(0 <= ss->ply && ss->ply < MAX_PLY);
 
-    (ss + 1)->excludedMove = bestMove = Move::none();
+    bestMove = Move::none();
     (ss + 2)->killers[0] = (ss + 2)->killers[1] = Move::none();
     (ss + 2)->cutoffCnt                         = 0;
     ss->multipleExtensions                      = (ss - 1)->multipleExtensions;