remove unused std imports

modules/gpu/src/cuda/optical_flow_farneback.cu

@@ -42,7 +42,6 @@
 
 #if !defined CUDA_DISABLER
 
-#include <stdio.h>
 #include "internal_shared.hpp"
 #include "opencv2/gpu/device/common.hpp"
 #include "opencv2/gpu/device/border_interpolate.hpp"
@@ -57,8 +56,6 @@
 #define BORDER_SIZE 5
 #define MAX_KSIZE_HALF 100
 
-using namespace std;
-
 namespace cv { namespace gpu { namespace device { namespace optflow_farneback
 {
     __constant__ float c_g[8];