fix remove_from_list when destroying for not null columns

basically, don't bother setting position to null

Change-Id: I560b129e499552d94f3cd4709a361432a54db627
Reviewed-on: https://gerrit.instructure.com/28367
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>