Revert "Add flatten_result_index_mapping field to Config (#1276) (#12…

[jackiehanyang]

This reverts commit d3bd162.

Description

[Describe what this change achieves]

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.61%. Comparing base (138dbd0) to head (f9bf688).
Report is 18 commits behind head on 2.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x    #1291      +/-   ##
============================================
+ Coverage     71.40%   77.61%   +6.21%     
- Complexity     4878     5435     +557     
============================================
  Files           518      532      +14     
  Lines         22931    23243     +312     
  Branches       2260     2303      +43     
============================================
+ Hits          16373    18041    +1668     
+ Misses         5509     4157    -1352     
+ Partials       1049     1045       -4     

Flag	Coverage Δ
plugin	77.61% <100.00%> (+6.21%)	⬆️

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

Files with missing lines	Coverage Δ
src/main/java/org/opensearch/ad/model/ADTask.java	97.99% <100.00%> (ø)
.../java/org/opensearch/ad/model/AnomalyDetector.java	86.41% <ø> (-0.61%)	⬇️
.../handler/AbstractAnomalyDetectorActionHandler.java	97.77% <100.00%> (+77.77%)	⬆️
...va/org/opensearch/forecast/model/ForecastTask.java	97.58% <100.00%> (ø)
...java/org/opensearch/forecast/model/Forecaster.java	79.51% <ø> (+8.43%)	⬆️
.../rest/handler/AbstractForecasterActionHandler.java	90.90% <100.00%> (+90.90%)	⬆️
...n/java/org/opensearch/timeseries/model/Config.java	85.71% <ø> (-1.31%)	⬇️

... and 109 files with indirect coverage changes