Clean-up DigitizationTests ART jobs
This merge request reduces the number of DigitizationTests ART jobs which run for the master nightly by making the following changes:
- Drop obosolete MC16 presampling configuration tests.
- Merge CA vs CG comparison jobs and regression check jobs for MC20 presampling.
Tagging @bwynne, @hasib, @tadej, @tlari, @tcuhadar
NB As these changes are purely to ART grid scripts, then they will not affect the CI result.