Revert "Allow as_variable(scalar)"
Revert "Allow as_variable(scalar)"

This reverts commit b54a37291b41a134ffeb2dd939e385abdcbc0b6a.

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