Update SaferCPPExpectations after 294045@main

https://bugs.webkit.org/show_bug.cgi?id=292003
rdar://149931392

Reviewed by Vitor Roriz.

After landing 294045@main, `loader/DocumentThreadableLoader.h` is no longer
failing the ForwardDeclChecker.

Update the expectations file.

* Source/WebCore/SaferCPPExpectations/ForwardDeclCheckerExpectations:
  Update file to remove `loader/DocumentThreadableLoader.h`

Canonical link: https://commits.webkit.org/294065@main

Author: zakariaridouh

Source/WebCore/SaferCPPExpectations/ForwardDeclCheckerExpectations

@@ -90,7 +90,6 @@ layout/formattingContexts/inline/text/TextUtil.cpp
 layout/integration/LayoutIntegrationUtils.h
 layout/integration/inline/LayoutIntegrationLineLayout.h
 layout/layouttree/LayoutElementBox.h
-loader/DocumentThreadableLoader.h
 loader/cache/CachedImageClient.h
 loader/cache/CachedResourceHandle.h
 page/DOMWindow.cpp