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/views/gcal-week-view.blp

@@ -16,7 +16,7 @@ template $GcalWeekView: Box {
       ToggleButton {
         visible: bind header.can-expand;
         active: bind header.expanded bidirectional;
-        tooltip-text: bind $get_header_state_tooltip(header.expanded) as <string>;
+        tooltip-text: bind $get_header_state_tooltip(header.expanded);
         hexpand: true;
         halign: center;
         valign: end;