drop authdir

<Black_Prince> halfline: what's the point of authdir in gdm? (/var/gdm)
?
<Black_Prince> it doesn't appear to be used anywere
<halfline> no point
<Black_Prince> there are some references in daemon/main.c, but nothing
ever gets stored in there
<Black_Prince> instead, xauthdir is used

https://bugzilla.gnome.org/show_bug.cgi?id=706974