Remove deleted feature flag [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

In gitlab-org/quality/triage-reports#1745 (closed), groupcompliance is being asked to remove the feature flag called usage_data_a_compliance_audit_events_api. ~~Per #284388 (comment 451148622), this feature flag will be removed by ~"group::product intelligence", not groupcompliance, so this changes the feature flag definition file to reflect.~~ Per !52947 (comment 566649084), this feature flag can be removed.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team