remove jwst.datamodels.schema submodule

[braingram]

jwst.datamodels.schema is an out-of-date duplicate of stdatamodels.schema

Regression tests running at: https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/1388/

Started another run given the age of the last one: https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/1427/ all failures in the above run are unrelated and the same as those on main: https://plwishmaster.stsci.edu:8081/job/RT/job/JWST/2877/

Checklist

• [ ] added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• [ ] updated relevant tests
• [ ] updated relevant documentation
• [ ] updated relevant milestone(s)
• [ ] added relevant label(s)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 66.26%. Comparing base (4d7c3a6) to head (5c193d5). Report is 25 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #175 +/- ## ========================================== + Coverage 64.84% 66.26% +1.42% ========================================== Files 103 101 -2 Lines 5694 5461 -233 ========================================== - Hits 3692 3619 -73 + Misses 2002 1842 -160 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.