removed unnecessary forward declaration, and added a comment about the remaining one.

git-svn-id: svn://bind10.isc.org/svn/bind10/branches/trac221b@2556 e5f2f494-b856-4b98-b285-d166d9295462

src/bin/auth/asio_link.h

@@ -30,10 +30,8 @@
 #include <exceptions/exceptions.h>
 
 namespace asio {
+// forward declaration forIOService::get_io_service() below
 class io_service;
-namespace ip {
-class address;
-}
 }
 
 class AuthSrv;