Setting a system wide default can be very handy but also completely messing up up your system. For instance changes to CSS files don't seem to work. Most catalog administrators forget to retract this privileges until it's to late.
Anyway here is how you can undo the damage.
As always be careful when editing directly into the catalog XML.. Backup!
Open the catalog in a off line mode:
Navigate to /system/metadata
Press Edit XML
Press again Edit
Carefully Delete edit the XML !
Press save, close catalog manager, restart presentation service.