Remove CHARSET=ascii from ISUPPORT

For one, [draft-brocklesby-irc-isupport-02][1] already defines "ascii" as the
default value. According to section 2 ("Except as
explicitly stated in its definition, a parameter should not be sent
unless it changes this default value, or the default value is vague,
badly defined, or differs between IRC server implementations"), there is
no point in sending it.

For another, [version 03 of the same draft][2] removes CHARSET ("It was
found to be unworkable;  a correct specification could not be devised to
represent its meaning across implementations."), and the token is not
present at all in [draft-hardy-irc-isupport-00][3].

[1]: https://tools.ietf.org/html/draft-brocklesby-irc-isupport-02#section-3.17
[2]: https://tools.ietf.org/html/draft-brocklesby-irc-isupport-03#section-4.8
[3]: https://tools.ietf.org/html/draft-hardy-irc-isupport-00