fix(app): remove exported Exec Element - it does not exist in this branch

src/app.zig

@@ -418,7 +418,6 @@ pub fn App(comptime E: type) type {
         pub const Element = element.Element(Event);
         pub const Alignment = element.Alignment(Event);
         pub const Scrollable = element.Scrollable(Event);
-        pub const Exec = element.Exec(Event, Queue);
         pub const Queue = queue.Queue(Event, 256);
     };
 }