core: Cancel nickname AutoWho on IrcUser quit

Remove any queued AutoWho for <nickname> if <nickname> quits.  This
avoids sending a needless AutoWho for <nickname> that'll only either
result in "No such nickname" if we're lucky, or random results for
non-extended WHO if <nickname> happens to match the realname/etc of
other IRC users.

This should fix any remaining cases of join/quit resulting in AutoWho
output in the Status Buffer for a network.