Allow the user to deregister a content type which is contributed by plugin.xml

05866463ef4920b75dfd7424a31644add49fbbf2 added the option to remove context type associations which the user created. A check in ContentTypeManager prevents the removal of non user extension.

Can this restriction be removed? A RCP client wants to remove an associating to the generic editor, and we currently have to modify the extension point registry to remove this association