Remove #pragma clang loop vectorize_width

Recent versions of Clang started warning when the loop doesn't get
vectorized, such as when compiling with -Oz (see bug).

To fix the build, remove the pragma and let the compiler decide on its
own when to vectorize.

Bug: chromium:1015665
Change-Id: I40a610c9e0d94cfd577a6cd2b01e6fdaa08bef7d
Reviewed-on: https://chromium-review.googlesource.com/c/libyuv/libyuv/+/1872580Reviewed-by: Frank Barchard <fbarchard@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>

source/row_common.cc

@@ -3227,9 +3227,6 @@ void NV12ToRGB565Row_AVX2(const uint8_t* src_y,
 float ScaleSumSamples_C(const float* src, float* dst, float scale, int width) {
   float fsum = 0.f;
   int i;
-#if defined(__clang__)
-#pragma clang loop vectorize_width(4)
-#endif
   for (i = 0; i < width; ++i) {
     float v = *src++;
     fsum += v * v;