[AIRFLOW-XXX] Remove flake8 from PR template

Since we have pre-commit and flake8 on CI there seems to be no
reason to keep this bullet point in PR template.

.github/PULL_REQUEST_TEMPLATE.md

@@ -31,7 +31,3 @@ Make sure you have checked _all_ steps below.
 - [ ] In case of new functionality, my PR adds documentation that describes how to use it.
   - 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](https://github.com/apache/airflow/blob/master/UPDATING.md) so we can assign it to a appropriate release
-
-### Code Quality
-
-- [ ] Passes `flake8`