Remove unused SubtractImagesStep

[jdavies-st]

Description

This PR removes the unused SubtractImagesStep which is old code that was completely replaced by BackgroundStep. This code is untested. And I believe unneeded?

Checklist

• Tests
• Documentation
• Change log
• Milestone
• Label(s)

[codecov]

Codecov Report

Merging #5919 (e356aab) into master (06e021f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5919      +/-   ##
==========================================
+ Coverage   77.86%   77.88%   +0.01%     
==========================================
  Files         400      400              
  Lines       35273    35295      +22     
==========================================
+ Hits        27465    27489      +24     
+ Misses       7808     7806       -2     

Flag	Coverage Δ		*Carryforward flag
nightly	77.86% <100.00%> (ø)		Carriedforward from 06e021f
unit	56.05% <100.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
jwst/lib/suffix.py	100.00% <ø> (+4.00%)	⬆️
jwst/step.py	100.00% <ø> (ø)
jwst/stpipe/integration.py	100.00% <ø> (ø)
jwst/background/__init__.py	100.00% <100.00%> (ø)

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