Stop trying to get "gtk-autojoin" as a string from chat rooms in the buddy

Stop trying to get "gtk-autojoin" as a string from chat rooms in the buddy
list.  This field is a boolean, and has been since at least 2004.  It looks
like it was a boolean even before then, too...
For some reason Nathan added this in revision
e9c6664819793a001faac32c865ed5d01d918bde with the comment:
"another one of those horrible hacks that I plan to remove in 6 months or so"

This causes this warning to be logged when right clicking on any chat room
where you have previously had auto-join checked, but it is now unchecked:
"(09:26:22) g_log: purple_blist_node_get_string: assertion purple_value_get_type(value) == PURPLE_TYPE_STRING' failed"

--HG--
extra : convert_revision : e4546fa88606ad39b1926afe342cdda4b92a2451