remove deprecated admin prefs

remove deprecated admin prefs

A new ikiwiki-transition moveprefs subcommand can pull the old data out of
the userdb and inject it into the setup file.

Note that it leaves the old values behind in the userdb too. I did this
because I didn't want to lose data if it fails writing the setup file for
some reason, and the old data in the userdb will only use a small amount of
space. Running the command multiple times will mostly not change anything.