Follow-up from "alpha: remove legacy attestation encoding in operations"

The following discussion from !13976 (merged) should be addressed:

• @dianegalloiswong started a discussion: (+1 comment)

  LGTM!

  Grepping "legacy" in proto_alpha has a lot of hits, but the only endorsement-related ones are in main.ml and that's normal, they are needed until the environment-updating MR.

  Grepping "endors" hits:

  • in lib_protocol, only a comment on old double signing events so it makes sense

  • in lib_client, an old TODO that's probably obsolete 🤷

  • in proto_alpha/README.md because the subdirectory list is completely out of date 🤷

  • in lib_delegate, the "endorse for" command; I imagine its removal is planned for a future MR, and anyway it's not blocking for snapshotting Q

  The protocol feels lighter already 🎉