Navigation: Try removing gray blobs.

[jasmussen]

Description

When you start fresh with a navigation block and insert an empty one, you get these gray blobs:

The initial idea was to let themes preinsert a navigation block at an appropriate place in the theme, but leave it otherwise unconfigured. With that goal in mind, the gray blobs were meant to help the placeholder state pass the "squint test", to look at least a little bit like the end result at least when the block was unselected.

Feedback suggested this didn't quite work as intended, though: the blobs looked like a "skeleton loading state", that they would be replaced with the real menu once loading had completed.

There's persistence in the navigation block now, with contents saved separately, and options for providing defaults and fallbacks. Depending on where that lands, we might be able to provide better first run experiences. In any case, it changes the equation from before, where an empty and unconfigured navigation block was more likely to appear in themes.

In light of this, it feels worth it to explore removing those blobs entirely, which this PR does:

As an added benefit, this simpler setup state entirely avoids a layout shift when selecting and unselecting.

Related PRs:

• Navigation: Try inheriting orientation in setup state. #36778
• Navigation: Try removing "Add all pages" from placeholder. #36775

How has this been tested?

Insert a navigation block and observe a setup state that shows "Navigation" when unselected, and "Navigation" with some options when selected.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[jasmussen]

It looks like I missed the fact that the PlaceholderPreview component is being used to indicate loading states in a few places. I think I'll try a different approach.

[jasmussen]

Closing in favor of #36858.