refactor(feature_flags configurations): remove defaults values

[ofekisr]

SUMMARY

Due to #15405 default values are redundant since partial dictionary can be merged to original dictionary

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #15425 (93c1dda) into master (d8a1acf) will decrease coverage by 0.25%.
The diff coverage is 100.00%.

❗ Current head 93c1dda differs from pull request most recent head 4cbc7d2. Consider uploading reports for the commit 4cbc7d2 to get more accurate results

@@            Coverage Diff             @@
##           master   #15425      +/-   ##
==========================================
- Coverage   77.23%   76.98%   -0.26%     
==========================================
  Files         975      975              
  Lines       50653    50651       -2     
  Branches     6223     6223              
==========================================
- Hits        39124    38994     -130     
- Misses      11318    11446     +128     
  Partials      211      211              

Flag	Coverage Δ
hive	?
mysql	81.73% <100.00%> (-0.01%)	⬇️
postgres	81.75% <100.00%> (-0.01%)	⬇️
presto	?
python	81.80% <100.00%> (-0.49%)	⬇️
sqlite	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/cli.py	51.47% <100.00%> (ø)
superset/config.py	97.02% <100.00%> (-0.02%)	⬇️
superset/utils/feature_flag_manager.py	95.00% <100.00%> (-0.24%)	⬇️
superset/db_engines/hive.py	0.00% <0.00%> (-82.15%)	⬇️
superset/db_engine_specs/hive.py	69.20% <0.00%> (-17.21%)	⬇️
superset/db_engine_specs/presto.py	83.36% <0.00%> (-6.95%)	⬇️
superset/db_engine_specs/sqlite.py	91.89% <0.00%> (-5.41%)	⬇️
superset/utils/celery.py	86.20% <0.00%> (-3.45%)	⬇️
superset/connectors/sqla/models.py	87.98% <0.00%> (-1.89%)	⬇️
superset/views/database/mixins.py	81.03% <0.00%> (-1.73%)	⬇️
... and 5 more

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[amitmiran137]

straight forward refactor 🚀