Editor: Restore Classic block in the inserter

[tyxla]

What?

Reverts the Classic block inserter-hiding work introduced in [62546] (#65166).

The Classic (core/freeform) block is once again available in the inserter by default. This removes:

• The wp_declare_classic_block_necessary() function and its enqueue_block_editor_assets action hook in script-loader.php / default-filters.php.
• The wp_classic_block_supports_inserter filter (which never shipped in a WordPress release).
• The associated PHPUnit tests in tests/phpunit/tests/dependencies/scripts.php.

Why?

The Classic block deprecation / inserter-gating effort is being reverted. The corresponding Gutenberg change is reverted in WordPress/gutenberg#79894. This PR keeps Core in sync by removing the backported PHP side.

Since the wp_classic_block_supports_inserter filter never made it into a WordPress release, it can be removed cleanly.

See WordPress/gutenberg#78067 (comment) for more info.

How?

This is a straight revert of [62546] — all 100 added lines across three files are removed. No other code referenced the function or filter.

Testing Instructions

1. Open the block editor and open the inserter — the Classic block should be available.
2. Confirm the window.__needsClassicBlock inline script is no longer printed on enqueue_block_editor_assets.
3. Run the dependencies/scripts test suite — the removed wp_declare_classic_block_necessary tests should no longer exist and the suite should pass.

See #65166.

🤖 Generated with Claude Code

[github-actions]

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Props tyxla, youknowriad, mukesh27.

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[github-actions]

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[tyxla]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 62652
GitHub commit: 76f9763