Enable on-hover flyout menus in sidebar of new navigation

What does this MR do and why?

Removes the super_sidebar_flyout_menus and enables the on-hover flyout menus in the left sidebar of new navigation by default.

👍 This feature flag is already enabled on gitlab.com since September 7.

See [Feature flag] Rollout of `super_sidebar_flyout... (#417237 - closed) and Bring back hovering (#415343 - closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

• On any page with a left sidebar (projects, groups etc.) hover a collapsed menu section (like "Plan", "Code").
• With the FF enabled, you'll see the flyout menus with all the section's items in it.
• Without the FF, sections have to be clicked to see their contents.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.