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/ImageObjectImpl.h

@@ -57,10 +57,6 @@ namespace ImageProcessingAtom
 
         bool CompareImage(const IImageObjectPtr otherImage) const override;
 
-        bool SaveImage(const char* filename, IImageObjectPtr alphaImage, AZStd::vector<AZStd::string>& outFilePaths) const override;
-        bool SaveImage(AZ::IO::SystemFileStream& out) const override;
-        bool SaveMipToFile(AZ::u32 mip, const AZStd::string& filename) const override;
-
         uint32_t GetTextureMemory() const override;
 
         EAlphaContent GetAlphaContent() const override;
@@ -79,7 +75,6 @@ namespace ImageProcessingAtom
         void SetNumPersistentMips(AZ::u32 nMips) override;
 
         void GlossFromNormals(bool hasAuthoredGloss) override;
-        void ConvertLegacyGloss() override;
         void ClearColor(float r, float g, float b, float a) override;
         //end virtual functions from IImageObject