Remove invalid-kwarg test for backward
Remove invalid-kwarg test for backward

chainer/chainer,niboshi/chainer,chainer/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,wkentaro/chainer,wkentaro/chainer,wkentaro/chainer,hvy/chainer,hvy/chainer,keisuke-umezawa/chainer,okuta/chainer,okuta/chainer,wkentaro/chainer,niboshi/chainer,niboshi/chainer,tkerola/chainer,chainer/chainer,pfnet/chainer,chainer/chainer,okuta/chainer,keisuke-umezawa/chainer,hvy/chainer,okuta/chainer,hvy/chainer,niboshi/chainer