Elevation Theme should not be available in core project installer

[stpaultim]

Description of the bug

The theme called Elevation has been removed due to licensing issues. It is no longer available on BackdropCMS.org, but it still shows up in the Backdrop project installer and throws an error when a user tries to download it.

Steps To Reproduce

1. Go to the theme installation page: admin/appearance/install
2. Go to the second page of themes and find "Elevation" - there is a message saying that it is no longer available, but the link to "Add to Installation Queue" is still available and works. It is possible to add it to the queue, but it all breaks when you try to download it.

Expected behavior

I don't think that this deleted project should show up at all.

Additional information

Wasn't sure which issue queue to put this on. But since it does not show up on BackdropCMS.org and it does show up in Backdrop the product, this seemed like a good place to start.

[klonos]

Unless there needs to be a more generic way that we treat such cases in the core Project Installer module, and how we present these projects or the wording of the messages, then I think that this may be an issue for https://github.com/backdrop-ops/backdropcms.org