clear out stale submission fields on resubmit

fixes CNVS-5604

Test plan:
  * set up an assignment that takes online_url and online_text_entry
    submissions
  * make a online_url submission for an assignment
  * have the same student make an online_text_entry submission
  * the api response should now *not* include the url anymore

Change-Id: I2389b0b3e8a565eed01f85ba2bed874c59585a2f
Reviewed-on: https://gerrit.instructure.com/37796
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>