Remove check that can never be true from RNNOp.
Remove check that can never be true from RNNOp.

Summary: As desc.

Reviewed By: salexspb

Differential Revision: D5971303

fbshipit-source-id: 4728b4df91e16c151efce48f1987f2e5d109f343

davinwang/caffe2,Yangqing/caffe2,pietern/caffe2,pietern/caffe2,caffe2/caffe2,xzturn/caffe2,davinwang/caffe2,Yangqing/caffe2,xzturn/caffe2,sf-wind/caffe2,sf-wind/caffe2,sf-wind/caffe2,sf-wind/caffe2,davinwang/caffe2,pietern/caffe2,Yangqing/caffe2,xzturn/caffe2,sf-wind/caffe2,Yangqing/caffe2,davinwang/caffe2,xzturn/caffe2,pietern/caffe2,Yangqing/caffe2,xzturn/caffe2,pietern/caffe2,davinwang/caffe2