(JP-3358) Delete undersampling_correction directories

[dmggh]

This PR deletes the directories for the undersampling_correction step. That step has been renamed to charge_migration
via JP-3337 and JP-3338.

Checklist for maintainers

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• Make sure the JIRA ticket is resolved properly

[hbushouse]

Regression test started at https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/879

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.09% 🎉

Comparison is base (270c04d) 74.38% compared to head (6b91788) 74.47%.

❗ Current head 6b91788 differs from pull request most recent head 7754e53. Consider uploading reports for the commit 7754e53 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7850      +/-   ##
==========================================
+ Coverage   74.38%   74.47%   +0.09%     
==========================================
  Files         459      459              
  Lines       37109    37064      -45     
==========================================
  Hits        27603    27603              
+ Misses       9506     9461      -45     

Flag	Coverage Δ		*Carryforward flag
nightly	77.37% <ø> (ø)		Carriedforward from 3e317d3

*This pull request uses carry forward flags. Click here to find out more.

Files Changed	Coverage Δ
jwst/ramp_fitting/ramp_fit_step.py	95.67% <0.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[hbushouse]

All CI failures are due to the use of the latest stdatamodels release, as opposed to stdatamodels/master.

[hbushouse]

There's an error in the way the new DQ flag name is used in the ramp_fit step right here:
https://github.com/spacetelescope/jwst/blob/master/jwst/ramp_fitting/ramp_fit_step.py#L462

That needs to be fixed. You can include it in this PR.

[hbushouse]

Another regtest run started at https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/881

[hbushouse]

Regtest results now show only expected differences, so I'm merging this.