[flow][cleanup] Fully cleanup traces

Summary:
An earlier cleanup doesn't touch the logic in this file, but just always hardcode the trace message to be `[]`. This diff finally cleans up the remaining logic.

Changelog: [internal]

Reviewed By: panagosg7

Differential Revision: D57167358

fbshipit-source-id: 8c206d8861d827207856684289eeda872801d0d4