Remove deprecated settings from code

[DarshitChanpura]

There are some lingering references to deprecated settings with messages that they will be deprecated in specific version, and these versions have long since released.

Example:
These audit config settings had been marked for removal in 2.0.0.0 but are still around in the code:

security/src/main/java/org/opensearch/security/auditlog/config/AuditConfig.java

Line 545 in d526c9f

public static final List<String> DEPRECATED_KEYS = ImmutableList.of(

Expected outcome:

• All deprecated settings that should be removed, have been removed

[cwperks]

[Triage] @DarshitChanpura Thank you for filing this issue. Since the settings are currently in the code, would it make sense to update the message on the 2.x line and remove the deprecated settings on main to be released in the 3.x line?