Case datepicker field conversion - Follow up cleanup

[eileenmcnaughton]

Overview

Cleanup following from #15614

Before

Newly redundant code still present

After

Newly redundant code gone

Technical Details

Comments

@seamuslee001 will see if tests hit the deprecated lines

[civibot]

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[seamuslee001]

Tests pass and i have tested that the search form doesn't generate the warnings, cleanup makes sense merging