output/httpd: remove broken DLNA support code

This code was added in 21851c0673 but
looks completely broken:

- the status code is "206 OK" but "206" would be "Partial Content"

- the "Content-Length" header has a bogus value

- the "Content-RangeX" parameter has different bogus values (why
  "Content-RangeX" anyway and not "Content-Range"?)

Apart from that, there are strange undocumented non-standard headers
which are probably there to work around bugs/expectations in one
broken proprietary client product.  But these days, MPD doesn't bend
over to support broken clients.  So let's kill this code.

Closes #304

src/output/plugins/httpd/HttpdClient.hxx

@@ -82,11 +82,6 @@ class HttpdClient final
 	 */
 	bool head_method;
 
-	/**
-         * If DLNA streaming was an option.
-         */
-	bool dlna_streaming_requested;
-
 	/* ICY */
 
 	/**