gui/blueprint: Drop unneeded casts to built-ins

Blueprint can now "infer" the type a closure returns, as long as it is
not the property of a custom GType, so some of the casts (e.g. `as
<string>`) will not be necessary anymore

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/733>

src/gui/event-editor/gcal-organizer-row.blp

@@ -3,7 +3,7 @@ using Adw 1;
 
 template $GcalOrganizerRow: Adw.ActionRow {
   title: _("Organizer");
-  subtitle: bind $generate_subtitle(template.organizer) as <string>;
+  subtitle: bind $generate_subtitle(template.organizer);
 
   styles [
     "property",