Revert "Fixed Alpha chanel detection when decoding images."

This reverts commit e6ede11.

Author: Olivier Poitrey

SDWebImage/SDWebImageDecoder.m

@@ -100,9 +100,7 @@ + (UIImage *)decodedImageWithImage:(UIImage *)image
     CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
     CGImageAlphaInfo alphaInfo = CGImageGetAlphaInfo(imageRef);
 
-    BOOL imageHasAlphaInfo = (alphaInfo != kCGImageAlphaNone &&
-                              alphaInfo != kCGImageAlphaNoneSkipFirst &&
-                              alphaInfo != kCGImageAlphaNoneSkipLast);
+    BOOL imageHasAlphaInfo = (alphaInfo != kCGImageAlphaNone);
 
     int bytesPerPixel = imageHasAlphaInfo ? 4 : 3;
     CGBitmapInfo bitmapInfo = imageHasAlphaInfo ? kCGImageAlphaPremultipliedLast : kCGImageAlphaNone;