Revert "deprecate is_linear"
Revert "deprecate is_linear"

This reverts commit 4da3839804ac2be0b440650d067f619dea16e316.

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