Remove JOIN_TAB::used_window_func, it is not used anymore

sql/sql_select.cc

@@ -19137,7 +19137,7 @@ int join_init_read_record(JOIN_TAB *tab)
 
   if (tab->distinct && tab->remove_duplicates())  // Remove duplicates.
     return 1;
-  if (tab->filesort && !tab->used_for_window_func && tab->sort_table())     // Sort table.
+  if (tab->filesort && tab->sort_table())     // Sort table.
     return 1;
 
   if (tab->select && tab->select->quick && (error= tab->select->quick->reset()))

sql/sql_select.h

@@ -430,8 +430,6 @@ typedef struct st_join_table {
   */
   Window_funcs_computation* window_funcs;
 
-  bool used_for_window_func;
-
   /**
     List of topmost expressions in the select list. The *next* JOIN TAB
     in the plan should use it to obtain correct values. Same applicable to