Remove `ci_register_job_service_one_by_one` feature flag
Description
Some time ago we added ci_register_job_service_one_by_one
feature flag to the Ci::RegisterJobService
to make builds queuing more efficient.
This has been successfully rolled out and this feature flag is now enabled by default.
Having it in the code results in one additional conditional branch and level of indirection. We should remove it to make it easier to implement #333114 (closed).
/cc @darbyfrey
Edited by Grzegorz Bizon