Refactor: delete dead SplitAndSizeRestrictions handling from FnApiDoFnRunner

[kennknowles]

The registrar only registers PAR_DO and
SPLITTABLE_PROCESS_SIZED_ELEMENTS_AND_RESTRICTIONS, so the SPLITTABLE_SPLIT_AND_SIZE_RESTRICTIONS cases are unreachable now that that stage has its own runner. Also collapse the two branches of the SDF-process consumer, which constructed identical objects regardless of whether windows were observed; the TODO for a non-window-observing variant is retained.

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[kennknowles]

R: @scwhittle

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.40%. Comparing base (d3ccb3a) to head (e9c37d5).
⚠️ Report is 27 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #39990      +/-   ##
============================================
+ Coverage     55.06%   58.40%   +3.33%     
- Complexity     1722    13548   +11826     
============================================
  Files          1075     2573    +1498     
  Lines        171589   268862   +97273     
  Branches       1262    11046    +9784     
============================================
+ Hits          94482   157021   +62539     
- Misses        74837   105896   +31059     
- Partials       2270     5945    +3675     

Flag	Coverage Δ
java	64.58% <100.00%> (-3.10%)	⬇️

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