old conversations: fix removing media comment

fixes CNVS-11322

test plan:
  * open old conversations
  * compose a message
  * add a media comment
  * click on the close icon next to your media comment
  * verify that the media comment is removed

Change-Id: I33ac6fe7cc4bf85b12ba7b0100a15fcc9eacb119
Reviewed-on: https://gerrit.instructure.com/30440
Reviewed-by: Jon Willesen <jonw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>