LibGUI: Remove Layout::try_add_spacer()

And fall back to the infallible add_spacer().

Work towards #20557.

Userland/Libraries/LibGUI/Widget.h

@@ -356,7 +356,7 @@ public:
     // In order for others to be able to call this, it needs to be public.
     virtual ErrorOr<void> load_from_gml_ast(NonnullRefPtr<GUI::GML::Node const> ast, UnregisteredChildHandler);
 
-    ErrorOr<void> add_spacer();
+    void add_spacer();
 
 protected:
     Widget();