game_list: Make ContainsAllWords an internally linked non-member function

This function actually depends on no internal class state, so it doesn't
even need to be a part of the class interface.

src/yuzu/game_list.h

@@ -89,7 +89,6 @@ private:
 
     void PopupContextMenu(const QPoint& menu_location);
     void RefreshGameDirectory();
-    bool ContainsAllWords(const QString& haystack, const QString& userinput) const;
 
     SearchField* search_field;
     GMainWindow* main_window = nullptr;