client/Internal: move client_domain to Domain.hxx

src/client/Client.cxx

@@ -17,14 +17,12 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
-#include "Internal.hxx"
+#include "Client.hxx"
 #include "Partition.hxx"
 #include "Instance.hxx"
 #include "util/Domain.hxx"
 #include "config.h"
 
-const Domain client_domain("client");
-
 Client::~Client() noexcept
 {
 	if (FullyBufferedSocket::IsDefined())