RAD-140: Remove the unused variance schema

[WilliamJamieson]

Resolves RAD-14

Closes #343

The variance schema is not used by anything, nor is it even supported by roman_datamodels

Checklist

• Schema changes discussed at RAD Review Board meeting
• Added entry in CHANGES.rst under the corresponding subsection
• Updated relevant roman_datamodels utilities and tests
• Passed romancal regression testing on Jenkins / PLWishMaster. Link: https://plwishmaster.stsci.edu:8081/job/RT/job/romancal/XXX/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b41cdd6) 95.32% compared to head (44c27d4) 95.32%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #344   +/-   ##
=======================================
  Coverage   95.32%   95.32%           
=======================================
  Files           4        4           
  Lines         171      171           
=======================================
  Hits          163      163           
  Misses          8        8           

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[PaulHuwe]

Have you confirmed that RDM will not need an update to match this removal?

[PaulHuwe]

LGTM

[WilliamJamieson]

Have you confirmed that RDM will not need an update to match this removal?

Yes, greping for variance indicates no mentions of it within the code base. Moreover, roman_datamodels is only directly aware of any tagged schemas and variance is not a tagged schema.