net: openvswitch: remove unnecessary ASSERT_OVSL in ovs_vport_del()

ASSERT_OVSL() in ovs_vport_del() is unnecessary because
ovs_vport_del() is only called by ovs_dp_detach_port() and
ovs_dp_detach_port() calls ASSERT_OVSL() too.

Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>