Remove GKEStartPodOperator when backporting

[turbaszek]

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Issue link: WILL BE INSERTED BY boring-cyborg

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[codecov-io]

Codecov Report

Merging #7908 into master will decrease coverage by 0.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #7908      +/-   ##
==========================================
- Coverage   87.06%   86.90%   -0.17%     
==========================================
  Files         929      932       +3     
  Lines       45028    45187     +159     
==========================================
+ Hits        39205    39270      +65     
- Misses       5823     5917      +94     

Impacted Files	Coverage Δ
...ud/example_dags/example_kubernetes_engine_setup.py	100.00% <100.00%> (ø)
airflow/kubernetes/volume_mount.py	44.44% <0.00%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0.00%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	47.18% <0.00%> (-45.08%)	⬇️
...viders/cncf/kubernetes/operators/kubernetes_pod.py	69.69% <0.00%> (-25.26%)	⬇️
airflow/kubernetes/refresh_config.py	50.98% <0.00%> (-23.53%)	⬇️
airflow/secrets/base_secrets.py	89.47% <0.00%> (-4.65%)	⬇️
airflow/www/views.py	76.58% <0.00%> (-0.12%)	⬇️
airflow/exceptions.py	100.00% <0.00%> (ø)
airflow/models/variable.py	93.33% <0.00%> (ø)
... and 21 more

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[mik-laj]

Hi. I thought about it longer and I think it should not be a problem to use the operators from airflow.contrib in Airflow 1.10. That way, we won't have to have a separate example.