AIRFLOW-3573 - Remove DagStat table

[ffinfo]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title.
  • https://issues.apache.org/jira/browse/AIRFLOW-3573

Description

The table DagStat can be removed after AIRFLOW-3561

Tests

No need, only removing code

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

No need, just removing unused code.

Code Quality

• Passes flake8

[codecov-io]

Codecov Report

Merging #4378 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4378      +/-   ##
==========================================
- Coverage   78.16%   78.14%   -0.03%     
==========================================
  Files         204      204              
  Lines       16530    16434      -96     
==========================================
- Hits        12921    12842      -79     
+ Misses       3609     3592      -17

Impacted Files	Coverage Δ
airflow/models/__init__.py	92.57% <ø> (-0.24%)	⬇️
airflow/jobs.py	77.44% <ø> (+0.02%)	⬆️
airflow/www_rbac/views.py	72.78% <ø> (+0.19%)	⬆️
airflow/www/views.py	69.47% <100%> (+0.15%)	⬆️
airflow/bin/cli.py	65.06% <100%> (+0.54%)	⬆️

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[Fokko]

LGTM Thanks @ffinfo

[kaxil]

Can you please resolve an error that was in the PR but because of broken Travis it wasn't detected:

======================================================================
3) FAIL: test_delete_dag_dag_still_in_dagbag (tests.api.common.experimental.test_delete_dag.TestDeleteDAGCatchError)
----------------------------------------------------------------------
   Traceback (most recent call last):
    tests/api/common/experimental/test_delete_dag.py line 61 in test_delete_dag_dag_still_in_dagbag
      delete_dag(self.dag_id)
   AssertionError: DagFileExists not raised

Please fix it in the new PR.

[Fokko]

@ffinfo PTAL

[ffinfo]

@Fokko I think meanwhile this is already fixed on master