bonding: remove unnecessary NULL check in bond_destructor

The free_percpu function also could check whether "rr_tx_counter"
parameter is NULL. Therefore, remove NULL check in bond_destructor.

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>