Fix obsolete reference to org.domokit.activity in hello_services (#6546)

See https://github.com/flutter/flutter/issues/6539
diff --git a/examples/hello_services/android/app/src/main/java/com/example/flutter/ExampleActivity.java b/examples/hello_services/android/app/src/main/java/com/example/flutter/ExampleActivity.java
index b336ea3..e3d64b0 100644
--- a/examples/hello_services/android/app/src/main/java/com/example/flutter/ExampleActivity.java
+++ b/examples/hello_services/android/app/src/main/java/com/example/flutter/ExampleActivity.java
@@ -17,7 +17,6 @@
 import android.widget.Button;
 import android.widget.TextView;
 
-import org.domokit.activity.ActivityImpl;
 import io.flutter.view.FlutterMain;
 import io.flutter.view.FlutterView;