Fix: remove signer crdt

[Ankarrr]

According to the discussion in https://github.com/farcasterxyz/protocol/discussions/179, we found there are old signer CRDT in the spec. This PR replaces the old description with the new one:

The new equivalent to "m.signer must be a valid Signer in the add-set of the Signer CRDT for message.fid" is "m.signer must be a valid key with Keystate.ADDED in the KeyRegistry contract for message.fid."