This feature flag was provided from ARC to runner container automatically to let it use `--ephemeral` instead of `--once` by default. As the support for `--once` is being dropped from the runner image via #1384, we no longer need that.
Ref #1196
The unregister timeout of 1 minute (no matter how long it is) can negatively impact availability of static runner constantly running workflow jobs, and ephemeral runner that runs a long-running job.
We deal with that by completely removing the unregistaration timeout, so that regarldess of the type of runner(static or ephemeral) it waits forever until it successfully to get unregistered before being terminated.
