[AIRFLOW-5179] Remove top level __init__.py

[ashb]

Make sure you have checked all steps below.

Jira

• https://issues.apache.org/jira/browse/AIRFLOW-5719

Description

• The recent commit 3724c2a ([AIRFLOW-4161] BigQuery to Mysql Operator #5711) to master introduced a init.py file in
  the project root folder, which basically breaks all imports in local
  development (pip install -e .) as it turns the project root into a
  package.

Tests

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

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.
  • 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