remove buggy default canvadoc base url

Test plan:
  * go to an account that has canvadocs configured
  * you should see the url you configured
  * go to another account that hasn't had canvadocs configured
  * there shouldn't be a base url filled in

Change-Id: I6889a02998f84a9ac7c6ac14d985beffea3a796c
Reviewed-on: https://gerrit.instructure.com/37412
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>