- upnp/LICENSE and LICENSE are the same files, put the content into
   COPYING (with whitespace stripped) which is automatically included in
   the distribution.
 - THANKS is also automatically distributed and doesn't need explicit
   mentioning.
 - README doesn't exist since commit aa343fa5.
 - TODO doesn't exist since 0629cfb7.
 - NEWS is old and so removed

This allows to drop doc_DATA completely. A side effect is that the
license and thanks files are not installed any more. But given that
installing the documentation was broken for quite some time already,
this is probably fine and nobody will notice it.