Document delegation removal in repository operations

[joshuagl]

@trishankatdatadog summarised the steps for removing a delegation in sigstore/root-signing#546 (comment). We really should include that in the repository operations section of the specification.

Copy/pasted here for ease of reference:

Seems right to me, yes. If there is a delegatee you want to delete, you should delete:

1. The targets only the delegatee is responsible for.
2. The delegatee's targets metadata.
3. The delegation off any delegator's targets metadata.

But keep the snapshot metadata about (2) around until timestamp/snapshot needs to be reset (e.g., due to a fast-forward attack, as described in Section 5.3.11 of the spec).

(3) can safely be updated in the snapshot metadata so long as it doesn't rollback itself.

Originally posted by @trishankatdatadog in sigstore/root-signing#546 (comment)

[shubhusion]

@joshuagl @trishankatdatadog assign this issue to me