[AIRFLOW-2227] Add delete method to Variable class

[jmsanders]

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Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title.

  • https://issues.apache.org/jira/browse/AIRFLOW-2227

Description

• Here are some details about my PR, including screenshots of any UI changes:

Add a delete method to the Variable class that no-ops if the Variable already does not exist. Additionally, refactor usages of session.query(Variable).filter(key).delete() to use the new Variable.delete(key) instead.

Tests

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

  • tests.core:CoreTest.test_variable_delete

Commits

• My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
  • All the public functions and the classes in the PR contain docstrings that explain what it does
  • If you implement backwards incompatible changes, please leave a note in the Updating.md so we can assign it to a appropriate release

Code Quality

• Passes flake8

[jmsanders]

The tests appear to be passing locally.

[Fokko]

We can remove the create_session here.

[codecov-io]

Codecov Report

Merging #4963 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4963      +/-   ##
==========================================
+ Coverage   75.74%   75.74%   +<.01%     
==========================================
  Files         458      458              
  Lines       29850    29853       +3     
==========================================
+ Hits        22609    22612       +3     
  Misses       7241     7241

Impacted Files	Coverage Δ
airflow/bin/cli.py	66.97% <100%> (-0.04%)	⬇️
airflow/models/__init__.py	92.94% <100%> (+0.01%)	⬆️

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[Fokko]

Thanks @jmsanders