Remove the Roman specific unit schema

[WilliamJamieson]

This is a follow-up to #220 and spacetelescope/roman_datamodels#131. Those PRs worked to deprecate the Roman-specific non-VOunits that were introduced to support serialization of the non-VOunits from astropy, which Roman needs to serialize. Since astropy/asdf-astropy#142, the non-VOunits can now be serialized directly by asdf-astropy.

Now that a new build of Roman is about to be released, all of the reference files will need to be regenerated. These files should now use the non-VOunit tag from asdf-astropy instead. Thus there is no need for the Roman-specific unit support in rad or roman_datamodels. This PR removes this support for rad.

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

Patch and project coverage have no change.

Comparison is base (8546809) 93.54% compared to head (a7dc13e) 93.54%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #248   +/-   ##
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  Coverage   93.54%   93.54%           
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  Files           4        4           
  Lines         124      124           
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  Hits          116      116           
  Misses          8        8           

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[kdupriestsci]

fine by me

[ddavis-stsci]

Looks good

[PaulHuwe]

LGTM