remove CV_64F conversion in tracking sample

modules/tracking/samples/tutorial_customizing_cn_tracker.cpp

@@ -123,7 +123,6 @@ void sobelExtractor(const Mat img, const Rect roi, Mat& feat){
     //! [sobel]
 
     //! [postprocess]
-    feat.convertTo(feat,CV_64F);
     feat=feat/255.0-0.5; // normalize to range -0.5 .. 0.5
     //! [postprocess]
 }

modules/tracking/tutorials/tutorial_customizing_cn_tracker.markdown

@@ -68,6 +68,6 @@ If you need a more detailed information to use @ref cv::Tracker, please refer to
 
 -#  **Post processing**
 
-    Make sure to convert the feature into @ref cv::CV_64F data format and normalize its value with range -0.5 to 0.5
+    Make sure to normalize the feature with range -0.5 to 0.5
 
     @snippet tracking/samples/tutorial_customizing_cn_tracker.cpp postprocess