Revert "add WebPBlendAlpha() function to blend colors against background"

This reverts commit dcbb1ca54a.

Dropping this for now to maintain compatibility for 0.3.1.

Change-Id: I44e032a072d317bb67e1439c42cff923e603038f

examples/tiffdec.c

@@ -100,7 +100,6 @@ int ReadTIFF(const char* const filename,
 #ifdef __BIG_ENDIAN__
       TIFFSwabArrayOfLong(raster, width * height);
 #endif
-      pic->use_argb = 1;
       ok = keep_alpha
          ? WebPPictureImportRGBA(pic, (const uint8_t*)raster, stride)
          : WebPPictureImportRGBX(pic, (const uint8_t*)raster, stride);