Remove protocol.mli

The following discussion from !4961 (merged) should be addressed:

• @g.b.fefe started a discussion: (+5 comments)

  Small questions while writing this MR:

  • what is the module Tezos_data is the protocol environment? If unused, should not it be removed from V6?

  • is the module Protocol really useful? It does not seem used in proto_alpha and according to the signature PROTOCOL in updater.mli the shell never provides or expects such value to/from the protocol. Should not it be removed from V6? It would avoid having to modify the type Protocol.env_version in each version of the protocol when adding a new protocol version. Or at least cannot this type be abstracted?