Make splitjob function static & private, Remove a couple of unused variables

[eileenmcnaughton]

The function is only called from one place but because it is on the Job BAO entity & is not static it is reliant on the previous function loading a Job BAO rather than using the saner & less leaky executeQuery - which I tried to change in #27303

[civibot]

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[colemanw]

retest this please