osbuilder: Drop Clear Linux

The Clear Linux rootfs is not being tested anywhere, and it seems Intel
doesn't have the capacity to review the PRs related to this (combined
with the lack of interested from the rest of the community on reviewing
PRs that are specific to this untested rootfs).

With this in mind, I'm suggesting we drop Clear Linux support and focus
on what we can actually maintain.

Signed-off-by: Fabiano Fidêncio <fabiano@fidencio.org>

docs/Developer-Guide.md

@@ -627,7 +627,7 @@ the following steps (using rootfs or initrd image).
 >
 > Look for `INIT_PROCESS=systemd` in the `config.sh` osbuilder rootfs config file
 > to verify an osbuilder distro supports systemd for the distro you want to build rootfs for.
-> For an example, see the [Clear Linux config.sh file](../tools/osbuilder/rootfs-builder/clearlinux/config.sh).
+> For an example, see the [Ubuntu config.sh file](../tools/osbuilder/rootfs-builder/ubuntu/config.sh).
 >
 > For a non-systemd-based distro, create an equivalent system
 > service using that distro’s init system syntax. Alternatively, you can build a distro

docs/design/architecture/guest-assets.md

@@ -135,7 +135,7 @@ See also the [process overview](README.md#process-overview).
 
 | Image type | Default distro | Init daemon | Reason | Notes |
 |-|-|-|-|-|
-| [image](background.md#root-filesystem-image) | [Clear Linux](https://clearlinux.org) (for x86_64 systems)| systemd | Minimal and highly optimized | systemd offers flexibility |
+| [image](background.md#root-filesystem-image) | [Ubuntu](https://ubuntu.com) (for x86_64 systems) | systemd | Fully tested in our CI |  systemd offers flexibility |
 | [initrd](#initrd-image) | [Alpine Linux](https://alpinelinux.org) | Kata [agent](README.md#agent) (as no systemd support) | Security hardened and tiny C library |
 
 See also: