Remove first_class_vulnerabilities feature flag

Proposal

When the First Class Vulnerabilities passed the MVC stage, the first_class_vulnerabilities feature flag has to be removed.

Related Issues:

• Docs: #34213 (closed)

Execution

• Remove the code branches corresponding to the "disabled" state of the feature flag
• Remove tests for the "disabled" state of the feature flag
• Remove comments from the code that have a TODO: reference to this issue