Fix database schema update introduced in [30121].

The index must be manually dropped before `dbDelta()` can add the new index
without throwing a notice.

Fixes #22023.
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Fix failing unit tests after [30096]: use `assertEqualSets()`.


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In `Custom_Image_Header->step_2()`, `$type` is set internally but never used.

See #30224.

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In `wp_xmlrpc_server->_prepare_comment()`, `$comment_date` is set internally but never used.

See #30224.

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In `wp_xmlrpc_server` methods, don't set `$blog_id` internally if it is never used, which is almost always. Updates the docs.

See #30224.

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In `wp_xmlrpc_server->pingback_ping()`, remove the internal variable `$way` each time it is set and not used.

See #30224.

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