common: Fix IrcUser::quitInternal not syncing quit

common: Fix IrcUser::quitInternal not syncing quit

Fix the new IrcUser::quitInternal() function to call IrcUser::quit()
over the network, restoring compatibility with both old and new
Quassel clients.

Only the core needs upgraded to fix this; the client does not sync
calls to IrcUser::quit().

> Before, core + client latest
<-- kiwipre-68 ([...]) has quit (Client Quit)
2020-07-11 17:53:26 [Warn ] "no matching slot for sync call: IrcUser::quitInternal (objectName=\"2/kiwipre-68\"). Params are:" ()

> After, core + client latest
IrcUser is successfully marked as having quit, removed from nick list.

Fixes the fix in 7298446b3e86140c4bbbfe7aeeb959c16c53363c