Remove broken link to PDF in cross-domain docs

Adobe have changed the location of the PDF. There is no gaurantee that
it will remain at the new location either. Since we already link to an
Adobe article about the cross-domain policy spec (that links to a PDF)
we don't need to link to both.

doc/crossdomain.md

@@ -14,8 +14,4 @@ custom headers in cross-domain requests, and are also used with sockets to
 grant permissions for socket-based connections.
 
 For full details, check out Adobe's article about the [cross-domain policy file
-specification](http://www.adobe.com/devnet/articles/crossdomain_policy_file_spec.html)
-
-Read the [Cross-domain policy file
-specification](http://learn.adobe.com/wiki/download/attachments/64389123/CrossDomain_PolicyFile_Specification.pdf?version=1)
-- (PDF, 129 KB)
+specification](http://www.adobe.com/devnet/articles/crossdomain_policy_file_spec.html).