Remove the feature flag npm_obtain_lease_to_create_package

Context

The feature flag npm_obtain_lease_to_create_package was enabled globally on Apr 24: link (internal)

What does this MR do and why?

This MR removes the feature flag npm_obtain_lease_to_create_package.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #397668 (closed)