ATOM-15086 Image Pipeline Unexpectedly Pre-Multiplying Alpha into Color Channels (#5358)

Applied the discard alpha in the begining of converting.

Also deleted some unused processing settings and steps.

Signed-off-by: Qing Tao <55564570+VickyAtAZ@users.noreply.github.com>

Gems/Atom/Asset/ImageProcessingAtom/Code/Source/Processing/ImageConvert.h

@@ -115,7 +115,6 @@ namespace ImageProcessingAtom
 
         //get output images
         IImageObjectPtr GetOutputImage();
-        IImageObjectPtr GetOutputAlphaImage();
         IImageObjectPtr GetOutputIBLSpecularCubemap();
         IImageObjectPtr GetOutputIBLDiffuseCubemap();
 
@@ -131,8 +130,6 @@ namespace ImageProcessingAtom
         //for alpha
         //to indicate the current alpha channel content
         EAlphaContent m_alphaContent;
-        //An image object to hold alpha channel in a separate image
-        IImageObjectPtr m_alphaImage;
 
         //output results of IBL cubemap generation, used in unit tests
         IImageObjectPtr m_iblSpecularCubemapImage;
@@ -171,9 +168,6 @@ namespace ImageProcessingAtom
         //convert to output color space before compression
         bool ConvertToOuputColorSpace();
 
-        //create alpha image if it's needed
-        void CreateAlphaImage();
-
         //pixel format convertion/compression
         bool ConvertPixelformat();