[AIRFLOW-7079] Remove redundant code for storing template_fields

[kaxil]

We don't need the logic in #6715 to store fields used in Extra Operator links as now we store all the template_fields (#7633)

---

Issue link: AIRFLOW-7079

Make sure to mark the boxes below before creating PR: [x]

• Description above provides context of the change
• Commit message/PR title starts with [AIRFLOW-NNNN]. AIRFLOW-NNNN = JIRA ID^*
• Unit tests coverage for changes (not needed for documentation changes)
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions.
• I will engage committers as explained in Contribution Workflow Example.

^* For document-only changes commit message can start with [AIRFLOW-XXXX].

---

In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.
Read the Pull Request Guidelines for more information.

[potiuk]

I understand that template_fields are now serialized by default ... Then this is perfectly OK

[potiuk]

However serialization tests are failing now :)

[codecov-io]

Codecov Report

Merging #7750 into master will decrease coverage by 22.33%.
The diff coverage is 98.33%.

@@             Coverage Diff             @@
##           master    #7750       +/-   ##
===========================================
- Coverage   86.96%   64.63%   -22.34%     
===========================================
  Files         915      914        -1     
  Lines       44188    44185        -3     
===========================================
- Hits        38429    28559     -9870     
- Misses       5759    15626     +9867

Impacted Files	Coverage Δ
...viders/google/cloud/operators/kubernetes_engine.py	97.18% <100%> (-0.08%)	⬇️
...oviders/google/cloud/utils/credentials_provider.py	91.83% <100%> (-1.39%)	⬇️
airflow/providers/qubole/operators/qubole.py	87.69% <100%> (-0.88%)	⬇️
airflow/utils/process_utils.py	27.55% <100%> (-45.71%)	⬇️
...rflow/providers/google/cloud/operators/bigquery.py	91.37% <100%> (-0.12%)	⬇️
airflow/providers/google/cloud/hooks/base.py	96.27% <97.29%> (+0.37%)	⬆️
...low/contrib/operators/wasb_delete_blob_operator.py	0% <0%> (-100%)	⬇️
airflow/contrib/hooks/vertica_hook.py	0% <0%> (-100%)	⬇️
airflow/contrib/sensors/__init__.py	0% <0%> (-100%)	⬇️
airflow/hooks/mssql_hook.py	0% <0%> (-100%)	⬇️
... and 500 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2a54512...2bfb401. Read the comment docs.