Revert "Use std::stable_sort() instead of std::sort()"

Unfortunatly std::stable_sort() implementation in gcc is
horrendously slow. We have a big performance regression on
Linux systems (-20% !)

So revert the commit and wait to fix the issue in a different
way, perhaps with an our home grown sorting, that should be
comparable in speed with std::sort()

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/move.h

@@ -62,7 +62,7 @@ struct MoveStack {
   int score;
 };
 
-// Note that operator< is set up such that std::stable_sort() will sort in descending order
+// Note that operator< is set up such that std::sort() will sort in descending order
 inline bool operator<(const MoveStack& f, const MoveStack& s) { return s.score < f.score; }