Devoices a selected nick on a channel. If nick is not given,
it will devoice you. If channel and nick are not given, it will
devoice you on all channels you're on, provided you have the rights
to.
By default, limited to AOPs or those with level 5 access and
above on the channel, or to VOPs or those with level 3 and above
for self devoicing.
