Use wfResetSessionID()

Now that core change I777f76ee has made wfResetSessionID available,
let's use it instead of duplicating the code ourselves.

Depends-On: I777f76ee8e2b953a1e972327bedc28e0ab1acf0d
Change-Id: I2c29df5aa445e39cc07b86d8308758aaf03b68b4