Remove todo about async multithreading

Turns out there is no async code that would benefit from being executed
on e.g. a multithreaded Tokio executor. Image loading is already being
run on GIO's threadpool, and async-sqlite already has its own thread on
which the connection lives. We also cannot easily bypass the latter and
do Rusqlite connection IO on a Tokio threadpool because Rusqlite
connections are not Send. Overall the current implementation is already
pretty optimal given that we're not exactly dealing with thousands of
operations per second in this little app.

src/main.rs

@@ -44,8 +44,6 @@ fn show_window ( application : & Application ) {
 	spawn_future_local ( async move {
 		async_result_context ! (
 			async {
-				// TODO: Move non-UI async tasks to non-local context
-
 				let data_manager = leak ( DataManager :: new () . await ? ) ;
 
 				let ui = UI :: new (