block.html.twig documents a "block" variable, which doesn't exist

Problem/Motivation

In core/modules/block/templates/block.html.twig (and the overriden ones in Classy and Bartik as well), there is the following variable documentation:

 * - block - The full block entity.
 *   - label_hidden: The hidden block title value if the block was
 *     configured to hide the title ('label' is empty in this case).
 *   - module: The module that generated the block.
 *   - delta: An ID for the block, unique within each module.
 *   - region: The block region embedding the current block.

Since #1989568: Remove block config ID from being used as an HTML ID or template suggestion that variable, however, does not exist anymore (see BlockViewBuilder::buildBlock() for proof).

Proposed resolution

Remove the documentation.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None

Beta phase evaluation