Deprecate the AJAX RenderElement

Problem/Motivation

I found that @RenderElement("ajax") (Drupal\Core\Render\Element\Ajax) is not used. What's is the purpose? Looks like this was the thing in Drupal 7 when we need to return plain JSON response without full page markup. But in Drupal 8 there are other and better ways to do this.

This render element not used by core and any contrib module. Looks like this is dead code.

Proposed resolution

• If the element is involved somewhere, it is necessary to write a more detailed comment to the element, because now it is not clear its purpose.
• If it is not used anywhere, and it looks like neither the core nor the contrib modules are using it, it should be removed, as it is a useless piece of code.

Remaining tasks

• Understand, why we have this element.
• Decide what to do with it.
• Do it :)

User interface changes

The interface won't get hurt in any way.

API changes

Looks like 0 API changes.

Data model changes

No data model changes.