Select2 for relations: previous content types are added to add new

Steps to reproduce

• Add two relations to a content type, each with one (different) target
• Edit an item of that content type
• Result:
  

The bug

• For the second relation the "Add new" drop down also contains the target content type of the first relation. This is incorrect.
• The "Add new" should be a drop down only if it has multiple target content types.