Code cleanup: remove WM subwindows.

These no longer made much sense after regions were added, they just
duplicated state that was already in the regions.

source/blender/windowmanager/intern/wm_operators.c

@@ -3310,7 +3310,7 @@ static void redraw_timer_step(
 			CTX_wm_area_set(C, sa_iter);
 
 			for (ar_iter = sa_iter->regionbase.first; ar_iter; ar_iter = ar_iter->next) {
-				if (ar_iter->swinid) {
+				if (ar_iter->visible) {
 					CTX_wm_region_set(C, ar_iter);
 					ED_region_do_draw(C, ar_iter);
 					ar_iter->do_draw = false;