Remove redirected group import/export topic from global navigation

What does this MR do and why?

The page at: https://docs.gitlab.com/ee/user/group/settings/import_export.html redirects to: https://docs.gitlab.com/ee/user/group/import/index.html, and we don't need separate navigation items anymore.

So we can remove one and have a single item for migrating groups.

CC @m_frankiewicz @wortschi

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