xen-netback: remove duplicated function definition

There are two duplicated xenvif_zerocopy_callback() definitions.
Remove one of them.
Signed-off-by: NLiang Li <liang.z.li@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/xen-netback/common.h

@@ -325,9 +325,6 @@ static inline pending_ring_idx_t nr_pending_reqs(struct xenvif_queue *queue)
 		queue->pending_prod + queue->pending_cons;
 }
 
-/* Callback from stack when TX packet can be released */
-void xenvif_zerocopy_callback(struct ubuf_info *ubuf, bool zerocopy_success);
-
 irqreturn_t xenvif_interrupt(int irq, void *dev_id);
 
 extern bool separate_tx_rx_irq;