we really should have validation for build_repository(): I resisted the urge to slap some validation code in between as I think it makes sense to make that consciously and use the same code in online-sign as well #343
there is no test for this since tuf-on-ci does not currently have CLI support for removing delegations (issue only came up since sigstore root-signing has removed delegations) -- this is not a big thing to add but it's not been needed so far...
Using snapshot contents is incorrect as snapshot is allowed to contain files that are no longer needed.
Fixes #342