Fix GH-12616: DOM: Removing XMLNS namespace node results in invalid default: prefix

The namespace data is freed and set to NULL, but there remain references
to the namespace declaration

Fix GH-12616: DOM: Removing XMLNS namespace node results in invalid default: prefix

The namespace data is freed and set to NULL, but there remain references
to the namespace declaration nodes. This (rightfully) confuses libxml2
because its invariants are broken. We also have to remove all remaining
references from the subtree. This fixes the data corruption bug.

Closes GH-12681.

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