Remove Identity FeatureFlag

[cwperks]

Description

This PR removes the identity feature flag.

The PR is a follow-up to 2 previous PRs:

• Add runAs to Subject interface and introduce IdentityAwarePlugin extension point #14630
• Remove identity-related feature flagged code from the RestController #15430

This PR is a prerequisite for opensearch-project/opensearch-plugins#238 to begin creating issues on individual plugin repos to adopt the new mechanism for system index interaction

There are 2 additional open PRs (1 in core repo and 1 in security repo) that need to be resolved before plugins start to adopt the new mechanism.

1. Allow Plugins to request to perform cluster actions and index actions with their assigned PluginSubject and prompt on install #15778
2. Implement new extension points in IdentityPlugin and add ContextProvidingPluginSubject security#4665

Related Issues

Resolves opensearch-project/security#4439

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for 1a07f98: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

❌ Gradle check result for 1a07f98: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for 1a07f98: SUCCESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.96%. Comparing base (2b670cc) to head (1a07f98).
Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #16024      +/-   ##
============================================
- Coverage     71.99%   71.96%   -0.04%     
- Complexity    64408    64420      +12     
============================================
  Files          5281     5281              
  Lines        301003   301000       -3     
  Branches      43483    43482       -1     
============================================
- Hits         216721   216624      -97     
- Misses        66522    66627     +105     
+ Partials      17760    17749      -11     

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