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 4863dea3("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>