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