Bug 1850342 - Remove non-standard, un-tested :-moz-loading pseudo-class. r=layout-reviewers,jfkthame

This is technically web-exposed, but if we needed to introduce it for
compat we could always re-introduce it matching false.

Differential Revision: https://phabricator.services.mozilla.com/D186938

dom/html/HTMLImageElement.cpp

@@ -341,8 +341,7 @@ void HTMLImageElement::AfterSetAttr(int32_t aNameSpaceID, nsAtom* aName,
 
   bool forceReload = false;
 
-  if (aName == nsGkAtoms::loading &&
-      !ImageState().HasState(ElementState::LOADING)) {
+  if (aName == nsGkAtoms::loading && !mLoading) {
     if (aValue && Loading(aValue->GetEnumValue()) == Loading::Lazy) {
       SetLazyLoading();
     } else if (aOldValue &&