remove unsupported retstep
remove unsupported retstep

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