[AIRFLOW-6714] Remove magic comments about UTF-8

[mik-laj]

# -*- coding: utf-8 -*- is a Python 2 thing. In Python 3+, the default encoding of source files is UTF-8 and that line is useless. In the next PR, I want to introduce automatic checking using pre-commit to avoid regression.

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Issue link: AIRFLOW-6714

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[codecov-io]

Codecov Report

Merging #7338 into master will increase coverage by 52.96%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #7338       +/-   ##
===========================================
+ Coverage   32.86%   85.82%   +52.96%     
===========================================
  Files         867      867               
  Lines       40560    40560               
===========================================
+ Hits        13331    34812    +21481     
+ Misses      27229     5748    -21481

Impacted Files	Coverage Δ
...low/contrib/operators/wasb_delete_blob_operator.py	100% <ø> (+100%)	⬆️
...rflow/contrib/hooks/gcp_video_intelligence_hook.py	100% <ø> (+100%)	⬆️
airflow/contrib/hooks/vertica_hook.py	100% <ø> (+100%)	⬆️
...irflow/example_dags/example_kubernetes_executor.py	85% <ø> (ø)	⬆️
...ample_dags/example_branch_python_dop_operator_3.py	75% <ø> (ø)	⬆️
airflow/contrib/utils/weekday.py	100% <ø> (+25%)	⬆️
...irflow/contrib/operators/redis_publish_operator.py	100% <ø> (+100%)	⬆️
airflow/contrib/hooks/qubole_hook.py	100% <ø> (+100%)	⬆️
airflow/contrib/sensors/qubole_sensor.py	100% <ø> (+100%)	⬆️
airflow/contrib/utils/gcp_field_sanitizer.py	0% <ø> (ø)	⬆️
... and 1067 more

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