full-bootstrap builder doesn't test anything

[Mark-Simulacrum]

It's clocking in at 20 minutes and seems to be doing nothing -- I suspect that this might be a result of @jyn514's changes to x.py defaults but have not confirmed.

We should fix this.

[jyn514]

(link to the PR: #73964)

[pietroalbini]

We discussed this during the infrastructure team meeting and agreed to remove the full-bootstrap builder. The builder doesn't provide much value for the stability of the project, as it rarely fails and we'd catch any failure it would prevent as soon as we promote nightly to beta.

[pietroalbini]

To fix this issue, the x86_64-gnu-full-bootstrap builders needs to be removed from the CI configuration and the Docker images directory.