fix(axevent): Allow undeclaring without callback

`apps/send_event/src/main.rs`:
- Consistently undeclare declarations and run event loop to work around
  a memory leak that would otherwise present in `ax_event_handler_new`.

Change-Id: Iaa8aeb67150c777e0bdc72cf5e2fa148035e93c8

apps/send_event/src/main.rs

@@ -163,8 +163,11 @@ mod tests {
     #[test]
     fn can_declare_without_callback() {
         let handler = Handler::new();
-        handler
+        let declaration = handler
             .declare::<fn(axevent::flex::Declaration)>(&topic().unwrap(), true, None)
             .unwrap();
+        handler.undeclare(&declaration).unwrap();
+        let main_context = glib::MainContext::default();
+        while main_context.iteration(false) {}
     }
 }

crates/axevent/src/flex.rs

@@ -317,8 +317,7 @@ impl Handler {
             self.declaration_callbacks
                 .lock()
                 .unwrap()
-                .remove(declaration)
-                .unwrap();
+                .remove(declaration);
         }
         result
     }