PackageRevision task list removal/reimplementation

[efiacor]

Remove the "Task List" concept from PackageRevision
#465

Remove the "Task List" concept from PackageRevision. The primary purpose for this is smarter updates, but we are not using it, it is not fully tested and likely has some bugs, and the implementation is not scalable (it leads to very large PackageRevision objects), and that complicates moving PackageRevisions to pure CRs (see below).
#450

[liamfallon]

See nephio-project/porch#140