Updating YAML does not properly handle changes to add / remove a challenge variable from a question

[fpayer]

The issue was that when I originally uploaded the challenge yaml, I had a text answer for the question. I subsequently switched the answer to use a variable. When I updated the yaml on the platform, the db entry for the question never removed the static text answer. If the check_answer function sees there is a static text answer, it compares against that and never actually uses the variable

Steps to reproduce:

1. Upload a YAML that uses a static value as the answer
2. Update the YAML to use a challenge variable as the answer
3. Attempt to make an answer submission - the old static value should be what is compared