fix cv v8 build error (#3278)

fix cv v8 build error, test=develop

lite/utils/cv/bgr_rotate.cc

@@ -75,7 +75,6 @@ void rotate90_hwc(const uint8_t* src, uint8_t* dst, int w_in, int h_in) {
   int64_t stride_h = 4 * win;
   int64_t stride_h_w = 4 * win - 24;
   int ww = w_out - 8;
-  [w_out * h_out * 3];
   // block 8*8. -- 8*8
   int i = 0;
   for (i = 0; i < h_in - 7; i += 8) {