Removing the job_id field from tasks

[daviddavis]

fixes #4507
https://pulp.plan.io/issues/4507

Please be sure you have read our documentation on creating PRs:
https://docs.pulpproject.org/en/3.0/nightly/contributing/pull-request-walkthrough.html

[codecov]

Codecov Report

Merging #30 into master will decrease coverage by 0.05%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
- Coverage    66.5%   66.45%   -0.06%     
==========================================
  Files          66       66              
  Lines        2962     2957       -5     
==========================================
- Hits         1970     1965       -5     
  Misses        992      992

Impacted Files	Coverage Δ
pulpcore/tasking/util.py	34.69% <0%> (ø)	⬆️
pulpcore/tasking/worker.py	0% <0%> (ø)	⬆️
pulpcore/app/response.py	100% <100%> (ø)	⬆️
pulpcore/app/serializers/task.py	100% <100%> (ø)	⬆️
pulpcore/tasking/tasks.py	43.33% <46.15%> (ø)	⬆️
pulpcore/app/models/task.py	68.22% <50%> (-0.59%)	⬇️

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