Remove the `nuget_package_registry` feature flag

What does this MR do?

Remove the nuget_package_registry feature flag.

The feature flag is currently globally enabled on GitLab.com and set default_enabled: true.

This is the last step of the rollout tracking issue: #205682 (closed)

Tip for reviewers: reviewing without whitespace differences might simplify your work :)

Note to me

• Once this MR is deployed on production, don't forget to properly clean up by running chatops command in #production channel.

Screenshots

n/a

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