Dry up whether super sidebar should be shown

What does this MR do and why?

Dry up whether super sidebar should be shown

This moves a few instances of current_user.use_new_navigation to use NavHelper.show_super_sidebar? instead. This has the benefit, that we can now change the logic in a single place (e.g. once the feature flag becomes the default).

Furthermore we are able to remove the super_sidebar_supported? private method and it's use, because all sidebars support the super sidebar since: !115162 (merged) is merged.

Fixes: Dry up show_super_sidebar? and gon.use_new_navi... (#391875 - closed) • Lukas Eipert • 16.0

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