Bug 1820634 - Remove nsTreeColFrame. r=Gijs

Layout-wise it doesn't have any special code. Only weirdness is:

 * Invalidation when size changes (we can do that with a
   ResizeObserver).

 * Weird paint retargetting to the splitter, which we can fix by using
   negative margins instead and using a regular splitter.

Differential Revision: https://phabricator.services.mozilla.com/D171768

layout/base/nsCSSFrameConstructor.cpp

@@ -201,8 +201,6 @@ static FrameCtorDebugFlags gFlags[] = {
 #  define NUM_DEBUG_FLAGS (sizeof(gFlags) / sizeof(gFlags[0]))
 #endif
 
-#include "nsTreeColFrame.h"
-
 //------------------------------------------------------------------
 
 nsIFrame* NS_NewLeafBoxFrame(PresShell* aPresShell, ComputedStyle* aStyle);
@@ -4132,7 +4130,6 @@ nsCSSFrameConstructor::FindXULTagData(const Element& aElement,
   static constexpr FrameConstructionDataByTag sXULTagData[] = {
       SIMPLE_XUL_CREATE(image, NS_NewXULImageFrame),
       SIMPLE_XUL_CREATE(treechildren, NS_NewTreeBodyFrame),
-      SIMPLE_XUL_CREATE(treecol, NS_NewTreeColFrame),
       SIMPLE_TAG_CHAIN(label,
                        nsCSSFrameConstructor::FindXULLabelOrDescriptionData),
       SIMPLE_TAG_CHAIN(description,