Remove 16.04 Godot engine from 20.04 images
This MR removes the xenial build of the Godot engine from the focal images for two reasons:
- it doesn't work on focal, because it links against libraries not present in focal
- the Godot engine from Ubuntu repos works on UT focal
See clickable#410 (closed) for details.
Edited by Jonatan Hatakeyama Zeidler