TUF: Remove support for non-bundled roots of trust

[woodruffw]

Per conversation in #542: the bundled trust root is now considered the stable interface for retrieving all of the various pieces of Sigstore's root of trust, so we can remove support for the "standalone" targets that it replaced.

We should do #580 first, to convince ourselves that we haven't introduced any bugs by switching over to the bundled trust root.

[asraa]

Hey! Just FYI I've created a staging repository that can be pushed to a new staging bucket (to keep backwards compatibility because the root keys are different) that is now update-able and contains the trusted_root.json

@haydentherapper can fill you in about what the URL of the bucket will be