Remove the deadlock_detection feature (#7497)

* related #7494 

Removes the `deadlock_detection` feature, since we now have a more
primitive panic-after-30s deadlock detection which works well enough and
even detects kinds of deadlocks that the `deadlock_detection` feature
never supported.

crates/egui_demo_app/tests/test_demo_app.rs

@@ -62,6 +62,9 @@ fn test_demo_app() {
                     .type_text("file://../eframe/data/icon.png");
 
                 harness.get_by_role_and_label(Role::Button, "✔").click();
+
+                // Wait for the image to load
+                harness.try_run_realtime().ok();
             }
             _ => {}
         }