app: remove outdated TODO of app termination

This was resolved in CL 11733. The "C.stopApp()" triggers the
termination of an app.

Change-Id: I48d4b798e42bc0c8b780ed5c0ec50968b62d0afa
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/369199
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>

Author: changkun

app/darwin_desktop.go

@@ -59,7 +59,6 @@ func main(f func(App)) {
 	go func() {
 		f(theApp)
 		C.stopApp()
-		// TODO(crawshaw): trigger runApp to return
 	}()
 
 	C.runApp()