Revert "conversations: fix last_authored_message_at"

fixes CNVS-14333

This reverts commit 4e83960eb4b5ba2f5bfa61d33ddbc3d3d896a745.

Change-Id: I4631088293abc5f2bcb281d60165f9710c7b6111
Reviewed-on: https://gerrit.instructure.com/38622
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>