Remove explicit setting of ptr at destruction
Remove explicit setting of ptr at destruction

ktnyt/chainer,wkentaro/chainer,aonotas/chainer,wkentaro/chainer,niboshi/chainer,hvy/chainer,anaruse/chainer,kikusu/chainer,benob/chainer,benob/chainer,tkerola/chainer,cemoody/chainer,jnishi/chainer,pfnet/chainer,cupy/cupy,okuta/chainer,niboshi/chainer,okuta/chainer,kiyukuta/chainer,chainer/chainer,delta2323/chainer,keisuke-umezawa/chainer,cupy/cupy,hvy/chainer,jnishi/chainer,ktnyt/chainer,ktnyt/chainer,ronekko/chainer,hvy/chainer,okuta/chainer,keisuke-umezawa/chainer,kikusu/chainer,wkentaro/chainer,chainer/chainer,niboshi/chainer,keisuke-umezawa/chainer,hvy/chainer,cupy/cupy,chainer/chainer,wkentaro/chainer,rezoo/chainer,kashif/chainer,chainer/chainer,niboshi/chainer,ysekky/chainer,keisuke-umezawa/chainer,ktnyt/chainer,cupy/cupy,jnishi/chainer,AlpacaDB/chainer,okuta/chainer,jnishi/chainer,AlpacaDB/chainer