irc: do not bridge CTCP commands other than ACTION

[patrickxia]

CTCP commands other than ACTION are designed for client-to-client interaction on IRC networks. Drop such messages when we receive them.

Also get rid of a "CTCP_ACTION" handler in the handler registration. The registration

1. can't do anything (if anything, we wanted the string constant girc.CTCP_ACTION, which is "ACTION")
2. doesn't do anything in this context, because CTCP handlers are registered separately: https://github.com/lrstanley/girc/blob/f47717952bf9258e02eac14f1b9723bcf084e618/ctcp.go#L205

The PRIVMSG handler already listens to all CTCPs.

Thanks to @lexande for the bug report (found on a live instance).

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[42wim]

Thanks 👍