[AIRFLOW-5572] Clear task reschedules when clearing task instances

[feluelle]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-5572
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.
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Description

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Currently when you clear tasks the clear mechanism won't delete reschedule entries related to the task instance from the db.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

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  4. Subject uses the imperative mood ("add", not "adding")
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  6. Body explains "what" and "why", not "how"

Documentation

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  • All the public functions and the classes in the PR contain docstrings that explain what it does
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[codecov-io]

Codecov Report

Merging #6217 into master will decrease coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6217      +/-   ##
==========================================
- Coverage   80.01%   79.93%   -0.09%     
==========================================
  Files         610      610              
  Lines       35176    35179       +3     
==========================================
- Hits        28147    28120      -27     
- Misses       7029     7059      +30

Impacted Files	Coverage Δ
airflow/models/taskinstance.py	93.78% <100%> (+0.03%)	⬆️
airflow/operators/postgres_operator.py	0% <0%> (-100%)	⬇️
airflow/hooks/postgres_hook.py	94.73% <0%> (-1.76%)	⬇️
airflow/utils/sqlalchemy.py	91.52% <0%> (-1.7%)	⬇️
airflow/hooks/dbapi_hook.py	86.44% <0%> (-1.7%)	⬇️
airflow/jobs/backfill_job.py	89.9% <0%> (-1.53%)	⬇️
airflow/utils/dag_processing.py	56.55% <0%> (+0.17%)	⬆️

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[feluelle]

Ready for re-Review :) @ashb @milton0825 @XD-DENG

Waiting for CI to get a green check mark :) - locally it breathed successfully