Remove the gutenberg-block-directory experimental flag

[tellyworth]

This removes the flag and its UI, enabling the block directory by default.

Note that it does not change the API namespace, which is still __experimental.

See #23384.

How has this been tested?

Manually tested, by verifying that the block directory is searched from the editor:

1. Visit Posts / Add New Post
2. Click Add Block
3. In Search For A Block, typeboxer, waves, or bliss.
4. Confirm that some blocks from the directory are shown, headed with the text "No blocks found in your library. These blocks can be downloaded and installed:"

Types of changes

• Removes conditional checks for the gutenberg-block-directory experiment.
• Removes the settings field.
• Removes the related setup from phpunit bootstrap.
• In short, enables the Block Directory by default.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• 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.

[ellatrix]

Looks like there's some e2e test failures.

[StevenDufresne]

I don’t have your fork setup @tellyworth but we’ll probably want to the remove the following lines:

gutenberg/packages/e2e-tests/specs/experiments/block-directory-add.test.js

Line 14 in bacb520

import { useExperimentalFeatures } from '../../experimental-features';

gutenberg/packages/e2e-tests/specs/experiments/block-directory-add.test.js

Line 176 in bacb520

useExperimentalFeatures( [ '#gutenberg-block-directory' ] );

[noisysocks]

Code looks good and the tests pass!