Remove feature flag for npm upload and always use temporary package

Context

In Use temporary package for npm upload (!211279 - merged) we have introduce the changes to npm Registry package upload to use a temporary file for package upload. The changes were added with the feature flag.

The feature flag was succesfuly enabled on gitlab.com for all users on 2026-01-05.

What does this MR do and why?

Remove packages_npm_temp_package feature flag for npm upload and always use temporary package.

References

• The feature flag rollout issue: [FF] `packages_npm_temp_package ` -- Use tempor... (#579369 - closed)
• Feature issue: https://gitlab.com/gitlab-org/gitlab/-/issues/578214+
• Follow up issue: Refactor create npm package service to always r... (#583459)

Screenshots or screen recordings

No.

How to set up and validate locally

The standard operations to publish and deprecate a package should continue working without issues.

Publish a new npm package

https://docs.gitlab.com/user/packages/npm_registry/#publish-to-gitlab-package-registry

Deprecate existing package version

https://docs.gitlab.com/user/packages/npm_registry/#deprecate-a-package

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #578214