6.4: Delete obsolete attachment object type

[BurntimeX]

The object type is need for the migration of existing assignments. ref bc94d77

<type>
	<name>com.woltlab.wcf.article</name>
	<definitionname>com.woltlab.wcf.attachment.objectType</definitionname>
	<classname>wcf\system\attachment\ArticleAttachmentObjectType</classname>
</type>