spec: disable last_authored_message_at checks

CNVS-13598 fixed some instances where last_authored_message_at was
not being set correctly. unfortunately, it caused regressions in
forwarding and reply-all. CNVS-14333 reverted CNVS-13598, but
left fragile last_authored_message_at checks.

this commit removes those checks. they should be put back when we
fix the original problem in CNVS-13598

Change-Id: Iaa04d8c3ee41e9753b9f8ea042e035b12cc619f6
Reviewed-on: https://gerrit.instructure.com/38680
Product-Review: Bryan Madsen <bryan@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>