virtio-net: Remove unused page size constant

The page_size constant is not used in the code so lets just remove it.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

drivers/virtio-net.cc

@@ -386,8 +386,6 @@ namespace virtio {
         }
     }
 
-    static const int page_size = 4096;
-
     void virtio_net::fill_rx_ring()
     {
         trace_virtio_net_fill_rx_ring(_ifn->if_index);