Remove unused private methods

[snuyanzin]

Description

I realised there is a bunch of private/package private unused methods...
Since they are private and unused this is a PR to remove them

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/5612/
• CommitID: 9837ce1
  Please examine the workflow log, locate, and copy-paste the failure below.
  Flakey test?
  Iterate to green.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/5613/
• CommitID: 391dab7

[codecov-commenter]

Codecov Report

Merging #4926 (391dab7) into main (8c9ca4e) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main    #4926      +/-   ##
============================================
+ Coverage     70.78%   70.81%   +0.02%     
+ Complexity    57906    57901       -5     
============================================
  Files          4689     4689              
  Lines        276991   276914      -77     
  Branches      40311    40302       -9     
============================================
+ Hits         196074   196097      +23     
+ Misses        64677    64583      -94     
+ Partials      16240    16234       -6     

Impacted Files	Coverage Δ
.../opensearch/gradle/info/GlobalBuildInfoPlugin.java	37.15% <ø> (+0.59%)	⬆️
...ain/java/org/opensearch/painless/antlr/Walker.java	85.01% <ø> (+0.18%)	⬆️
...admin/indices/segments/IndicesSegmentResponse.java	73.86% <ø> (+8.86%)	⬆️
...ava/org/opensearch/cluster/node/DiscoveryNode.java	89.83% <ø> (+3.68%)	⬆️
...main/java/org/opensearch/index/engine/Segment.java	71.42% <ø> (+5.47%)	⬆️
.../opensearch/index/mapper/DocumentMapperParser.java	90.14% <ø> (+7.02%)	⬆️
...org/opensearch/index/query/ExistsQueryBuilder.java	93.54% <ø> (+15.42%)	⬆️
...ain/java/org/opensearch/ingest/IngestMetadata.java	98.14% <ø> (+5.16%)	⬆️
...ava/org/opensearch/snapshots/SnapshotsService.java	47.50% <ø> (+0.67%)	⬆️
...a/org/opensearch/test/OpenSearchIntegTestCase.java	57.17% <ø> (+1.21%)	⬆️
... and 472 more

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[dblock]

@nknize Any underwater mines you know of with this?