Remove the container layer workaround and use the rhel94 image directly from registry when running bootc image builder.

This is blocked by RHEL-34807

The workaround was introduced in the https://github.com/openshift/microshift/pull/3318 PR.

See the `test/image-blueprints/layer5-bootc/group0/rhel94-bootc-base.containerfile` and `test/image-blueprints/layer5-bootc/group1/rhel94-bootc.image-bootc` files.