Remove files from the tflite_files list that should not be shared. (#340)

* Remove files from the tflite_files list that should not be shared.

This was added with
https://github.com/tensorflow/tflite-micro/pull/134/.

However, the correct approach would be to only pull in the portable
implementations from:
 * https://github.com/tensorflow/tensorflow/blob/be7a4de6adfbd303ce08be4332554dff70362612/tensorflow/lite/kernels/internal/reference/portable_tensor_utils.cc
 * https://github.com/tensorflow/tensorflow/blob/be7a4de6adfbd303ce08be4332554dff70362612/tensorflow/lite/kernels/internal/reference/portable_tensor_utils.h
 * https://github.com/tensorflow/tensorflow/blob/be7a4de6adfbd303ce08be4332554dff70362612/tensorflow/lite/kernels/internal/reference/portable_tensor_utils_impl.h

* Added the portable_tensor_utils to the files to sync.

ci/tflite_files.txt

@@ -58,6 +58,8 @@ tensorflow/lite/kernels/internal/reference/mul.h
 tensorflow/lite/kernels/internal/reference/neg.h
 tensorflow/lite/kernels/internal/reference/pad.h
 tensorflow/lite/kernels/internal/reference/pooling.h
+tensorflow/lite/kernels/internal/reference/portable_tensor_utils.cc
+tensorflow/lite/kernels/internal/reference/portable_tensor_utils_impl.h
 tensorflow/lite/kernels/internal/reference/prelu.h
 tensorflow/lite/kernels/internal/reference/process_broadcast_shapes.h
 tensorflow/lite/kernels/internal/reference/quantize.h
@@ -81,7 +83,6 @@ tensorflow/lite/kernels/internal/min.h
 tensorflow/lite/kernels/internal/portable_tensor.h
 tensorflow/lite/kernels/internal/strided_slice_logic.h
 tensorflow/lite/kernels/internal/tensor_ctypes.h
-tensorflow/lite/kernels/internal/tensor_utils_common.h
 tensorflow/lite/kernels/internal/types.h
 tensorflow/lite/kernels/kernel_util.h
 tensorflow/lite/kernels/padding.h