Don't require a transaction to mark a participant up-to-date

Summary:
Pathway to D17685. We no longer have "behindTransactionPHID", so we no longer need the latest transaction.

This allows some code to be removed.

Test Plan:

• Grepped for callsites to markUpToDate() and variables used in the calls.
• Sent messages in a couple threads, viewed them, saw unread counts go away.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D17733