Remove some pointless code:

- Remove the CHARSET entry from ISUPPORT.

  CHARSET was removed in draft-brocklesby-irc-isupport-03 and we
  always used the default value anyway. This has also been removed
  in the latest version of Charybdis.

- Remove irc::sockets::satouser.

  This helper method was longer than the code it replaced.
Purge docs/rfc from the repository.

These are of no use to 99% of users and anyone who actually wants
to read them should be capable of using Google to find them.