CAP CLEAR and client->server CAP ACK are no longer valid

[jesopo]

https://ircv3.net/specs/core/capability-negotiation#errata

Previous versions of this specification referred to a CAP CLEAR command, which has been removed because it is not useful. We do not recommend implementing or supporting CAP CLEAR. See issue #134 for more information, including rationale for this clarification.

Previous versions of this spec listed that CAP ACK could be sent from the client to server, for capabilities that required extra client acknowledgement. This was removed since cap modifiers have been deprecated and removed (except for -, which has been specified in other ways).

[jesopo]

CAP CLEAR has already been removed from charybdis charybdis-ircd/charybdis@ceabbbb