Remove deprecated import-related topics from global navigation

What does this MR do and why?

Related to: gitlab!109291 (merged), we want to reduce the visibility of deprecated features.

These are the pages we no longer want to link to:

• https://docs.gitlab.com/ee/user/project/import/phabricator.html
• https://docs.gitlab.com/ee/user/project/import/gitlab_com.html
• https://docs.gitlab.com/ee/raketasks/import.html

CC @wortschi @m_frankiewicz

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