8df776e367
In https://github.com/projectatomic/rpm-ostree/pull/1484
AKA commit 235e8f82da
we introduced a new `rojig` treefile entry that is nicer and
more maintainable than a split spec file.
In prep for more "pure rojig" work, drop support for this key.
Note we can also only now do this because we hard require Rust, and
the new model is only implemented in Rust.
The only known user of this today is FAHC, but that is now
[pinned to an assembler build](https://pagure.io/fedora-atomic-host-continuous/c/cdcb6ae943bee643983d4bd22d00030f2eb930d2?branch=master).
Closes: #1558
Approved by: jlebon