Remove info note for package authentication

What does this MR do and why?

Describe in detail what your merge request does and why.

With #333444 (closed) resolved, this MR removes the info note at the bottom of https://docs.gitlab.com/ee/user/packages/package_registry/index.html#authenticate-with-the-registry doc page.

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Screenshot of the doc page before the change

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Related to #333444 (closed)