let users delete the last profile link, fixes CNVS-10912

test plan:
1. enable profiles (if necessary)
2. edit your profile
3. add some links
4. delete all the links
5. confirm they get deleted

Change-Id: Ia4dc35556475c3971f12ebe91f67e4bfd6c2a92c
Reviewed-on: https://gerrit.instructure.com/29559
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Dorman <ddorman@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>