R1:c96e17f5ea42

Remove "behindTransactionPHID" from ConpherenceParticipant

Summary: Pathway to D17685. Nothing reads this field and it has no use or value.

Test Plan:
- Ran migration.
- Grepped for `behindTransactionPHID`.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D17729