ext/workspaces: fix appearing of hidden workspaces

Do this by removing the needs-update tracking. The gain
was questionable to begin with and turns out it  doesn't
work correctly with multiple outputs.

include/modules/ext/workspace_manager.hpp

@@ -136,8 +136,6 @@ class Workspace {
   Gtk::Button button_;
   Gtk::Box content_;
   Gtk::Label label_;
-
-  bool needs_updating_ = false;
 };
 
 }  // namespace waybar::modules::ext