Remove the support for new WooCommerce Navigation

[eason9487]

User story

As a dev, I would like to make the codebase cleaner when something is no longer in need.

Is your feature request related to a problem?

Considering the new WooCommerce Navigation:

• Its options had been set to uneditable
  • Ref: Disable the new navigation in the WC Admin. woocommerce#36456
• Planned to sunset this feature
  • Ref: Advanced settings > Features: remove Navigation setting when store isn't using it woocommerce#44224

  • We'll sunset this feature when WP phase 3 is deployed more broadly.

• Its option is hidden for a store that doesn't have it enabled
  • Advanced settings > Features: remove Navigation setting when store isn't using it woocommerce#45800
• Started being removed
  • [Navigation Feature] Remove code strategically woocommerce#48997
• It also gradually loses its functionality in WooCommerce Core.
  • Duplicate item warings
  • A weird "Extensions0" item is listed in the Extensions menu, which is the same link as the bottom one. Surprisingly, it's the Extensions Marketplace page and it's considered as an "extension" as well.

How to reproduce the problem

Run wp option update woocommerce_navigation_enabled 'yes'

Describe the solution you'd like

Clean up the code related to the new WooCommerce Navigation.