Revert "Add flatten_result_index_mapping field to Config (#1317)"

[jackiehanyang]

This reverts commit cbe1f33.

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 80.17%. Comparing base (9d3bb72) to head (bb12ebe).
Report is 1 commits behind head on 2.18.

Additional details and impacted files

@@            Coverage Diff            @@
##               2.18    #1350   +/-   ##
=========================================
  Coverage     80.16%   80.17%           
+ Complexity     5687     5684    -3     
=========================================
  Files           533      533           
  Lines         23430    23409   -21     
  Branches       2335     2333    -2     
=========================================
- Hits          18783    18768   -15     
+ Misses         3545     3544    -1     
+ Partials       1102     1097    -5     

Flag	Coverage Δ
plugin	80.17% <ø> (+<0.01%)	⬆️

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	98.00% <ø> (-0.01%)	⬇️
.../java/org/opensearch/ad/model/AnomalyDetector.java	89.34% <ø> (+0.35%)	⬆️
.../handler/AbstractAnomalyDetectorActionHandler.java	97.82% <ø> (-0.05%)	⬇️
...va/org/opensearch/forecast/model/ForecastTask.java	97.99% <ø> (-0.01%)	⬇️
...java/org/opensearch/forecast/model/Forecaster.java	79.88% <ø> (+1.39%)	⬆️
.../rest/handler/AbstractForecasterActionHandler.java	91.11% <ø> (-0.20%)	⬇️
...n/java/org/opensearch/timeseries/model/Config.java	86.73% <ø> (+0.42%)	⬆️

... and 8 files with indirect coverage changes