unregister_nav_menu() should remove_theme_support()

[kovshenin]

I thought about this when reading through #26276. When a child theme (or a plugin) unregisters all menu items, current_theme_supports still returns true for menus, which can cause certain weirdness with the UI:

• If widgets are not supported, the Menus menu is still visible
• The "your theme has no menus but you can use a widget" text is not displayed on the Menus screen
• The "Theme locations" label is visible on the Menus screen but the list is empty

Since register_nav_menu() adds theme support, I think unregister_nav_menu() should remove it.

[kovshenin]

26900.diff​ removes theme support during unregister_nav_menu() + unit tests.

[mordauk]

Shouldn't users still be able to create custom menus and add them via widgets even if the theme does not explicitly support menus?

[kovshenin]

Replying to mordauk: Sure. If widgets are supported by the theme, the menus UI will be shown regardless of navigation menu support.

[mordauk]

Replying to kovshenin:

Replying to mordauk: Sure. If widgets are supported by the theme, the menus UI will be shown regardless of navigation menu support.

Ah, didn't realize that. I just assumed the menus UI got disabled when the theme didn't support it.

[nacin]

In 27220:

Remove theme support for 'menus' in unregister_nav_menu() when there are no more menus.

props kovshenin.
fixes #26900.

[nacin]

In 29894:

Fix theme support unit test added in [27220].

The test assumed that the active theme either had no menu locations, or a subset of 'primary' and 'secondary'. see #26900.