Remove parsing of panel.order attribute in userInputSpec.xml, require panel.id instead

The panel.order attribute in userInputSpec.xml doesn't make any sense. The documentation says:
This is the order number of the user input panel for which this specification should be used. Counting starts at 0 and increments by 1 for each instance of the user input panel. If a spec should be used for the second occurrence of the user input panel use order="1".
Actually, the order of user input panels are given by the order they are defined within

<panels>...</panels>

and addtionally depending on panel conditions they might depend on. That's really sufficient and explicitly assigning an order to the panel definitions is ambigous.