remove gpu code when backward mkldnn_softmax

paddle/gserver/activations/MKLDNNActivation.cpp

@@ -189,27 +189,19 @@ Error __must_check MKLDNNSoftmaxActivation::forward(Argument& act) {
 Error __must_check MKLDNNSoftmaxActivation::backward(Argument& act) {
   MatrixPtr outputV = act.value;
   MatrixPtr outputG = act.grad;
-
-  if (outputG->useGpu()) {
-    outputG->softmaxBackward(*outputV);
-  } else {
-    SetDevice device(act.deviceId);
   Matrix::resizeOrCreate(sftMaxDot_,
                          outputG->getHeight(),
                          outputG->getWidth(),
                          /* trans */ false,
-                           useGpu(act.deviceId));
+                         /* useGpu */ false);
   Matrix::resizeOrCreate(sftMaxSum_,
                          outputG->getHeight(),
                          1,
                          /* trans */ false,
-                           useGpu(act.deviceId));
-
+                         /* useGpu */ false);
   sftMaxDot_->dotMul(*outputG, *outputV);
   sftMaxSum_->colMerge(*sftMaxDot_);
-
   act.grad->softmaxDerivative(*act.value, *sftMaxSum_);
-  }
   return Error();
 }