net: thunderx: remove set but not used variable 'tail'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/ethernet/cavium/thunder/nicvf_queues.c: In function nicvf_sq_free_used_descs:
drivers/net/ethernet/cavium/thunder/nicvf_queues.c:1182:12: warning:
 variable tail set but not used [-Wunused-but-set-variable]

It's not used since commit 4863dea3fab01("net: Adding support for Cavium ThunderX network controller"),
so remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zheng zengkai <zhengzengkai@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/cavium/thunder/nicvf_queues.c

@@ -1179,13 +1179,12 @@ void nicvf_sq_disable(struct nicvf *nic, int qidx)
 void nicvf_sq_free_used_descs(struct net_device *netdev, struct snd_queue *sq,
 			      int qidx)
 {
-	u64 head, tail;
+	u64 head;
 	struct sk_buff *skb;
 	struct nicvf *nic = netdev_priv(netdev);
 	struct sq_hdr_subdesc *hdr;
 
 	head = nicvf_queue_reg_read(nic, NIC_QSET_SQ_0_7_HEAD, qidx) >> 4;
-	tail = nicvf_queue_reg_read(nic, NIC_QSET_SQ_0_7_TAIL, qidx) >> 4;
 	while (sq->head != head) {
 		hdr = (struct sq_hdr_subdesc *)GET_SQ_DESC(sq, sq->head);
 		if (hdr->subdesc_type != SQ_DESC_TYPE_HEADER) {