This reverts commit 40fcf5a633.
This commit triggers bug that we haven't fully tracked down, where web
app clients will continually send `update_message_flags` requests,
that then send out via the events system "0 messages were marked as
read" notices, eventually leading to a load spike.
The Tornado part can likely be fixed by checking if
updated_message_ids is empty, but we need to track down the frontend
bug as well.
We were blindly adding / removing flag from UserMessages without
check if they even need to be updated.
This caused server to repeatedly update flags for messages which
already had been updated, creating a confusion for other clients
like mobile.
Fixes#22164