Fix warnings: method is overriding a synchronized method without being synchronized

Eclipse Version 2022-06 (4.24.0) reports this warning in the following places:

```
The method FXDnDInteropN.FXDragGestureRecognizer.setComponent(Component) is overriding a synchronized method without being synchronized FXDnDInteropN.java line 187 /swing/src/main/java/com/sun/javafx/embed/swing/newimpl
The method PrismImage.deref() is overriding a synchronized method without being synchronized PrismImage.java line 64 /web/src/main/java/com/sun/javafx/webkit/prism
The method SeparateThreadTimer.notifyTick() is overriding a synchronized method without being synchronized Timer.java line 167 /web/src/main/java/com/sun/webkit
```