DSL: remove support for `nested_container`

Obsoleted by new form `container :nested`.

This PR only removes DSL support; the job is incomplete.
Internally, `container :nested` is still implemented as a
pseudo-artifact named `nested_container`.  The intention
is to factor nested containers out of artifacts.
Merge pull request #7451 from rolandwalker/remove_nested_container

DSL: remove support for `nested_container`