chore: Remove unused GH workflows attestation.yml

We currently call the GH action kubewarden/github-actions/actions/attestation.yaml.

The path to this GHA is embedded in the resulting OIDC Sigstore URI
signature. Consuming the workflow that we are removing here would change
the OIDC Sigstore URI signature, which is a breaking change and also
would need business logic in `slsactl`.

Signed-off-by: Víctor Cuadrado Juan <vcuadradojuan@suse.de>

Author: viccuad