Customize: Formally deprecate unused Customizer classes.

The `WP_Customize_New_Menu_Control` and `WP_Customize_New_Menu_Section` PHP classes and `wp.customize.Menus.NewMenuControl` JS class were deprecated in 4.9 through [42034] with the intention of removing them in 5.0. Since more time has passed than originally intended, this change leaves `WP_Customize_New_Menu_Control` and `WP_Customize_New_Menu_Section` to prevent any potential backwards compatibility issues, and formally deprecates them. The `wp.customize.Menus.NewMenuControl` JS class is removed.

Props weston.ruter, matthias-reuter, jrf.
Fixes #42364.
Built from https://develop.svn.wordpress.org/trunk@46637


git-svn-id: https://core.svn.wordpress.org/trunk@46437 1a063a9b-81f0-0310-95a4-ce76da25c4cd