Retire Position::move_is_legal()

Use the new contains() method of struct MoveList

No functional change.

src/movegen.h

@@ -46,6 +46,10 @@ struct MoveList {
   bool end() const { return cur == last; }
   Move move() const { return cur->move; }
   size_t size() const { return last - mlist; }
+  bool contains(Move m) const {
+    for (const MoveStack* it(mlist) ; it != last; ++it) if (it->move == m) return true;
+    return false;
+  }
 
 private:
   MoveStack mlist[MAX_MOVES];