Customize: Remove deprecated nav menu classes

[westonruter]

The following classes are deprecated in 4.9 and can be removed in 5.0:

• wp.customize.Menus.NewMenuControl (in JS)
• WP_Customize_New_Menu_Section (in PHP)
• WP_Customize_New_Menu_Control (in PHP)

These classes are not used by plugins and they are not used in core. For more, see #42357.

In 42034:

Customize: Deprecate nav menu classes that are no longer used, instead of removing them immediately.

• Deprecate PHP classes WP_Customize_New_Menu_Section and WP_Customize_New_Menu_Control.
• Deprecate JS class wp.customize.Menus.NewMenuControl.
• Also introduce wp.customize.Menus.createNavMenu() for logic to create nav menus separately from the logic for handling UI interactions.

Amends [41768].
See #40104, #42364.
Fixes #42357.