Remove unused code to automatically select Slurm reservation

Description

We're no longer using a Slurm reservation for calibration jobs - we have partitions (upex-high & upex-middle) to give calibration priority instead. This removes some unused code for xfel-calibrate to automatically decide whether to use a reservation or not.

We're now using the --slurm-partition option from the webservice, which overrides this anyway, and we're not using the --priority option according to caldeploy configs on max-exfl016.

How Has This Been Tested?

CI should exercise the relevant code paths.

Types of changes

• Removal of feature no longer needed

Checklist:

• My code follows the code style of this project.

Reviewers

@danilevc @ahmedk