Remove CallWhenProcFinishes class in utils.py

[masayukig]

This commit removes CallWhenProcFinishes class in utils.py, its usage
was removed in the commit[1].

[1] 182c6ac

[codecov-commenter]

Codecov Report

Merging #328 (4be8393) into main (f819157) will increase coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #328      +/-   ##
==========================================
+ Coverage   60.88%   60.99%   +0.11%     
==========================================
  Files          30       30              
  Lines        2646     2623      -23     
  Branches      472      470       -2     
==========================================
- Hits         1611     1600      -11     
+ Misses        908      896      -12     
  Partials      127      127              

Flag	Coverage Δ
unittests	60.99% <ø> (+0.11%)	⬆️

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Impacted Files	Coverage Δ
stestr/utils.py	75.00% <ø> (+9.92%)	⬆️

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[mtreinish]

LGTM, I don't really remember what this was used for or why exactly I removed its usage ~5 years ago. But we can definitely remove this if it's not being used anymore.