delete web conference participants prior to deleting a web conference

so we don't leave orphaned data around

Change-Id: I183dea150b1f9d700dce0d74b9829222d0448031
Reviewed-on: https://gerrit.instructure.com/30402
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>