Remove support for unused custom User serialization

[nibix]

Description

As determined in #5200, OpenSearch 3 no longer needs to have the custom User serialization code which was introduced in #2802. That's because the last version of OpenSearch that used custom serialization was 2.13. OpenSearch 3 no longer supports mixed clusters with OpenSearch 2.13 - the minimum mixed cluster version is OpenSearch 2.19. Thus, the code in question is no longer necessary.

This PR removes the unneccesary code.

This is part of the preparation for the immutable user object PR #5212 which aims to remove a couple of special subclasses of the User object and thus breaks tests in the custom serialization code.

• Category
  • Maintenance
• Why these changes are required?
  • Code hygiene, maintenance facilitation
• What is the old behavior before changes and new behavior after changes?
  • No behavioral changes

Issues Resolved

Fixes #5200

Testing

• Int tests

Check List

• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 3 lines in your changes missing coverage. Please review.

Project coverage is 71.84%. Comparing base (7421138) to head (7723f55).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
.../org/opensearch/security/support/Base64Helper.java	93.33%	0 Missing and 2 partials ⚠️
...earch/security/auditlog/impl/AbstractAuditLog.java	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5339      +/-   ##
==========================================
- Coverage   72.03%   71.84%   -0.19%     
==========================================
  Files         384      380       -4     
  Lines       23927    23677     -250     
  Branches     3656     3636      -20     
==========================================
- Hits        17235    17011     -224     
+ Misses       4856     4846      -10     
+ Partials     1836     1820      -16     

Files with missing lines	Coverage Δ
.../main/java/com/amazon/dlic/auth/ldap/LdapUser.java	85.71% <ø> (-3.58%)	⬇️
...ava/org/opensearch/security/auth/UserInjector.java	88.88% <ø> (-0.65%)	⬇️
...org/opensearch/security/filter/SecurityFilter.java	66.02% <ø> (-0.80%)	⬇️
...urity/ssl/transport/SecuritySSLRequestHandler.java	62.50% <ø> (-9.46%)	⬇️
...g/opensearch/security/support/ConfigConstants.java	95.23% <ø> (ø)
.../org/opensearch/security/support/HeaderHelper.java	68.42% <100.00%> (-1.58%)	⬇️
...ensearch/security/support/SourceFieldsContext.java	46.87% <ø> (-10.91%)	⬇️
...search/security/transport/SecurityInterceptor.java	75.94% <100.00%> (-1.71%)	⬇️
...rch/security/transport/SecurityRequestHandler.java	54.72% <100.00%> (-0.31%)	⬇️
...c/main/java/org/opensearch/security/user/User.java	67.14% <ø> (-2.27%)	⬇️
... and 2 more

... and 7 files with indirect coverage changes

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[cwperks]

Thank you @nibix , overall the PR lgtm and is a nice simplification in the code.