Bug 1797308 - Make <xul:spacer> use non-XUL layout. r=jwatt

Nothing overrides its display value so it would get -moz-box display by
default, like all other XUL elements.

Differential Revision: https://phabricator.services.mozilla.com/D160222

layout/base/nsCSSFrameConstructor.cpp

@@ -4068,7 +4068,6 @@ nsCSSFrameConstructor::FindXULTagData(const Element& aElement,
 
   static constexpr FrameConstructionDataByTag sXULTagData[] = {
       SIMPLE_XUL_CREATE(image, NS_NewImageBoxFrame),
-      SIMPLE_XUL_CREATE(spacer, NS_NewLeafBoxFrame),
       SIMPLE_XUL_CREATE(treechildren, NS_NewTreeBodyFrame),
       SIMPLE_XUL_CREATE(treecol, NS_NewTreeColFrame),
       SIMPLE_TAG_CHAIN(button, nsCSSFrameConstructor::FindXULButtonData),