Remove Manifest logic and code

[tiegz]

in the quest to phase out RepositoryDependency, this removes any views or logic around Manifests. Manifests aren't used at all outside of RepositoryDependency code.

Why?

Because data in the Manifest table are disconnected from data in the Projects and Dependency tables, which we want to use instead of RepositoryDependency:

flowchart BT
Project --> Repository
Dependency --> Project
Manifest --> Repository
RepositoryDependency --> Manifest