Remove core_security_mr_widget feature flag

What does this MR do?

Remove core_security_mr_widget feature flag

Addresses #249550 (closed), part of &4393 (closed).

Screenshots (strongly suggested)

There's no visual change, but this is the UI this flag was previously guarding:

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