refactor: remove unused flatten function

[zhaoyongjie]

SUMMARY

The flatten operation after pivot operator has been replaced with flatten operator, so removed unused codes.

the flatten operator is here

No functionality should be impacted.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A

TESTING INSTRUCTIONS

CI

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• 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

[stephenLYZ]

LGTM

[codecov]

Codecov Report

Merging #20582 (94dcc3c) into master (b99482d) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #20582      +/-   ##
==========================================
- Coverage   66.80%   66.79%   -0.01%     
==========================================
  Files        1754     1754              
  Lines       65568    65561       -7     
  Branches     6933     6933              
==========================================
- Hits        43804    43793      -11     
- Misses      20010    20014       +4     
  Partials     1754     1754              

Flag	Coverage Δ
hive	53.77% <100.00%> (+0.01%)	⬆️
javascript	51.82% <ø> (-0.01%)	⬇️
mysql	82.36% <100.00%> (-0.01%)	⬇️
postgres	82.43% <100.00%> (-0.01%)	⬇️
presto	53.63% <100.00%> (+0.01%)	⬆️
python	82.86% <100.00%> (-0.01%)	⬇️
sqlite	82.21% <100.00%> (-0.01%)	⬇️
unit	50.64% <100.00%> (-0.02%)	⬇️

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

Impacted Files	Coverage Δ
...t-ui-chart-controls/src/operators/pivotOperator.ts	100.00% <ø> (ø)
...controls/src/operators/timeComparePivotOperator.ts	100.00% <ø> (ø)
...superset-ui-core/src/query/types/PostProcessing.ts	100.00% <ø> (ø)
superset/utils/pandas_postprocessing/__init__.py	100.00% <ø> (ø)
superset/utils/pandas_postprocessing/pivot.py	90.00% <ø> (-1.18%)	⬇️
superset/utils/pandas_postprocessing/utils.py	96.15% <100.00%> (-0.40%)	⬇️
...preset-chart-deckgl/src/layers/Hex/controlPanel.ts	50.00% <0.00%> (-50.00%)	⬇️
...reset-chart-deckgl/src/layers/Grid/controlPanel.ts	50.00% <0.00%> (-50.00%)	⬇️
...chart-deckgl/src/layers/Screengrid/controlPanel.ts	50.00% <0.00%> (-50.00%)	⬇️
...et-chart-deckgl/src/layers/Polygon/controlPanel.ts	33.33% <0.00%> (-16.67%)	⬇️

---

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 b99482d...94dcc3c. Read the comment docs.