You can set flow.log to /dev/null:
Add options to define path to workflow log and to disable logging (flow.log) · Issue #20100 · nextcloud/server · GitHub
occ config:app:set workflowengine logfile --value=/dev/null
I think then you can also delete flow.log. Please show if you really do not need the logfile.
Also you can set the loglevel in config/config.php for minimize logging.