ixgb: remove unused variable

The variable num_group_tail_writes is initialized but never used otherwise.

The semantic patch that makes this change is as follows:
(http://www.emn.fr/x-info/coccinelle/)

// <smpl>
@@
type T;
identifier i;
constant C;
@@

(
extern T i;
|
- T i;
  <+... when != i
- i = C;
  ...+>
)
// </smpl>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/net/ixgb/ixgb_main.c

@@ -2088,14 +2088,12 @@ ixgb_alloc_rx_buffers(struct ixgb_adapter *adapter)
 	struct ixgb_buffer *buffer_info;
 	struct sk_buff *skb;
 	unsigned int i;
-	int num_group_tail_writes;
 	long cleancount;
 
 	i = rx_ring->next_to_use;
 	buffer_info = &rx_ring->buffer_info[i];
 	cleancount = IXGB_DESC_UNUSED(rx_ring);
 
-	num_group_tail_writes = IXGB_RX_BUFFER_WRITE;
 
 	/* leave three descriptors unused */
 	while(--cleancount > 2) {