Remove unnecessary SetDefaultDevice(nullptr)
Remove unnecessary SetDefaultDevice(nullptr)

(new context session reset default device to nullptr)

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