Retire 'mate in x' hack

Sometimes is faster, but not always and on very long mates
produces strange scores probably due to truncation of PV
artifacts.

So simply perform normal search also in case of UCI 'mate x'
command, with the only difference that when a mate in x is
found search returns immediately.

No functional change.

src/search.cpp

@@ -602,11 +602,6 @@ namespace {
                  ss->staticEval, ss->evalMargin);
     }
 
-    // Handling of UCI command 'mate in x moves'. We simply return if after
-    // 'x' moves we still have not checkmated the opponent.
-    if (PvNode && !RootNode && !inCheck && Limits.mate && ss->ply > 2 * Limits.mate)
-        return eval;
-
     // Update gain for the parent non-capture move given the static position
     // evaluation before and after the move.
     if (   (move = (ss-1)->currentMove) != MOVE_NULL