Fix helpserv whine when helpers leave the channel; fix ?devoiceall

Remove the parting modeNode before making callbacks, so callbacks
don't get confused and think the user is still in the channel.

In ?devoiceall, OpServ should only remove voice from users who have it.
git-archimport-id: srvx@srvx.net--2004-srvx/srvx--devo--1.3--patch-64