Remove terminally deprecated JavaFX GTK 2 library

The JavaFX GTK 2 library was deprecated for removal as of JavaFX 18 by JDK-8273089. We should now remove this library from JavaFX.

This should be done as follows:

1. Remove the glassgtk2 library (libglassgtk2.so) from the JavaFX SDK
2. Print a warning message at runtime if the jdk.gtk.version property is set to 2
3. Maybe print a warning message at runtime if another native library has already loaded the GTK 2 library (since an application cannot load both the GTK 2 and GTK 3 libraries in the same process, the attempt to load gtk3 will fail)

This will need a CSR and a release note.