Update system in enable when

[MariaNHN]

When changing system, the enable when still have the old system values.

To reproduce:

1. Create a new form
2. Add an Alternativer element with some choices (e.g. Red and Blue)
3. Add a Beskjed element, and fill in some text
4. Add Betinget visning on the Beskjed. Eg. it should be displayed when Red is selected
5. Change the System value
6. See in json (printscreen below), this will also give an error msg. when trying to send in the form