RemoveTargetsWithExpires does not pin to a preferred delegation

[asraa]

repo.RemoveTargetsWithExpires will remove a target from ALL delegated target metadata: there is no way to specify a certain targets or delegation role.

This means that if a delegation had signed off on the same path as another (allowed), or one of top-level that takes precedence, then removing the target in the delegaiton (intended) will remove it everywhere.

cc @joshuagl detected this in sigstore root-signing
sigstore/root-signing#410

[rdimitrov]

Closing since the code base changed and this is no longer valid.

Thanks for raising this 👍