CI: delete the .buildkite/rootfs_images directory

[DilumAluthge]

This PR deletes the .buildkite/rootfs_images directory from the JuliaLang/julia repository.

I have already copied the contents of the .buildkite/rootfs_images directory to the JuliaCI/rootfs-images repository.

Here is my reasoning for this change:

1. The release assets are already hosted in the JuliaCI/rootfs-images repository, so I think it makes sense for the code to live there.
2. Having the code live in the JuliaCI/rootfs-images repository makes it easier to go from release asset to the code that was used to generate that release asset, and vice versa.
3. The GitHub Actions CI in the JuliaCI/rootfs-images repository allows us to automate the build process of the rootfs images, instead of needing to do it manually.
4. The GitHub Actions CI in the JuliaCI/rootfs-images repository generates valid rootfs images, in contrast to other setups (like the Docker setup I tried out with v1.1). For example, it fixed the issue I was having in #41606).

Note that even though we are moving the code for generating rootfs images to an external repository, this doesn't change the fact that in the JuliaLang/julia repository, we will still pin each pipeline to a specific version of a rootfs images by specifying both the rootfs_url and rootfs_treehash in a file that is checked into source control in the JuliaLang/julia repository.

Therefore:

1. Our Buildkite builds will still be reproducible, because we specify the rootfs_treehash.
2. Different permanent branches of Julia (master and release-*) can still use different versions of the rootfs images.