Remove unused LinearPipeline class

[eslavich]

This removes the LinearPipeline class, which is a simplified base class for pipelines whose steps can feed output into one another without requiring additional Python glue code. In practice all pipelines have required at least some glue, so the simplified base class was never used. I'd like to remove it so that we don't have to maintain it in the new stpipe package.

I'll create a ticket to remove the pars-linearpipeline reference type from crds.

[codecov]

Codecov Report

Merging #5590 (507c52e) into master (93b8d5d) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5590      +/-   ##
==========================================
+ Coverage   74.39%   74.43%   +0.04%     
==========================================
  Files         416      417       +1     
  Lines       38088    38175      +87     
  Branches     5637     5637              
==========================================
+ Hits        28336    28417      +81     
- Misses       9752     9758       +6     

Flag	Coverage Δ		*Carryforward flag
nightly	75.93% <100.00%> (ø)		Carriedforward from 28f5fae
unit	54.49% <100.00%> (-0.07%)	⬇️

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

Impacted Files	Coverage Δ
jwst/stpipe/utilities.py	95.37% <ø> (+0.37%)	⬆️
jwst/lib/suffix.py	100.00% <100.00%> (+4.00%)	⬆️
jwst/pipeline/__init__.py	100.00% <100.00%> (ø)
jwst/stpipe/__init__.py	100.00% <100.00%> (ø)
jwst/combine_1d/combine1d.py	88.64% <0.00%> (-0.38%)	⬇️
jwst/datamodels/__init__.py	100.00% <0.00%> (ø)
jwst/datamodels/multicombinedspec.py	66.66% <0.00%> (ø)

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[jdavies-st]

🎉

[hbushouse]

LGTM. Just needs a change log entry before merging.