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/Tests/ImageProcessing_Test.cpp

@@ -988,7 +988,6 @@ namespace UnitTest
             ASSERT_TRUE(process->IsSucceed());
 
             SaveImageToFile(process->GetOutputImage(), "rgb", 10);
-            SaveImageToFile(process->GetOutputAlphaImage(), "alpha", 10);
 
             process->GetAppendOutputProducts(outProducts);