Remove backward tests in python which are equivalent with C++ backward tests
Remove backward tests in python which are equivalent with C++ backward tests

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