Fix arginfo violation in removeChild() (#14717)

It was possible to return false without throwing an exception.
This is even wrong in "old DOM" because we expect either a NOT_FOUND_ERR

Fix arginfo violation in removeChild() (#14717)

It was possible to return false without throwing an exception.
This is even wrong in "old DOM" because we expect either a NOT_FOUND_ERR
or NO_MODIFICATION_ALLOWED_ERR according to the documentation.
A side effect of this patch is that it prioritises NOT_FOUND_ERR over
NO_MODIFICATION_ALLOWED_ERR but I think that's fine.

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