selftests: bonding: delete unnecessary line

"ip link set dev "$devbond1" nomaster"
This line code in bond-eth-type-change.sh is unnecessary.
Because $devbond1 was not added to any master device.

Signed-off-by: Liang Li <liali@redhat.com>
Acked-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>