bonding: netpoll: remove unwanted slave_dev_support_netpoll()

The __netpoll_setup() will check the slave's flag and ndo_poll_controller just
like the slave_dev_support_netpoll() does, and slave_dev_support_netpoll() was
not used by any place, so remove it.

Cc: Jay Vosburgh <fubar@us.ibm.com>
Cc: Veaceslav Falico <vfalico@redhat.com>
Cc: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>