Question item placeholder text is displayed in results view, looks like user input

How to reproduce

• Create a QTI2 essay or FIB question (maybe others also affected)
• In the question, add placeholder texts
• Take the test as student, don't fill anything into the fields
  => NOK: in results view that you get after the test (or in assessment tool), the placeholder text is visible. It is very difficult to distinguish the placeholder text from actual user answers.

Solution: don't add the placeholder attribute on disabled elements.