Remove upstream, mutators and validators from Repository API specification

[ciaranjohnston]

The porch API spec for a Repository:

https://github.com/nephio-project/porch/blob/cb1b392069de7711bb29c3524d6fc464e991f82a/api/porchconfig/v1alpha1/types.go#L56

includes parameters for "upstream", "mutators" and "validators". Upstream indicates a repository that may be upstream of this one, while mutators and validators are lists of functions to be evaluated any time a package is changed within the repository. These parameters are not currently used and not documented, so should be removed from the API specification. They can be reintroduced as and when a use case requires them.

This issue is a result of a discussion around this user story proposal:
https://docs.google.com/document/d/1d4USe4nOcOQAb0ULCMGSPtC72f0dbV0SguPCPEGYaac/edit?tab=t.0

[ciaranjohnston]

In kpt files, mutators and validators are defined in the kpt file. These are additional mutators and validators that are executed on every package in addition to the ones defined in the kpt file, although these are not currently supported.

[ciaranjohnston]

Check with @efiacor that this is a straightforward issue