Workflow no longer stores any user information

We stripped the user information out of Workflow, but forgot
to remove it from the UserMerger which was being written at
the same time.  Remove lingering references to workflow_user_id
and friends.

Bug: T78422
Change-Id: I1c2039be3c92d089a68acfdab4d459841879d7db
Merge "Workflow no longer stores any user information"