Add a mechanism to remove leftovers from tails::builder.

tails::builder does not provide a way to set `$ensure` to absent for all
its resources.
Rather than implementing it in the class supposed to be removed, let's
just transition by removing leftovers in the new tails::iso_builder
manifest. It also removes leftovers from the tails::website_builder
classe which are not needed anymore on Jenkins isobuilders.

This commit can be reverted once we decide no more tails::builder
manifests are in use.

Refs: #12505