Remove CSS regions support, keeping a bare minimum to support "region-based" multicol.

Source/core/rendering/RenderRegionSet.cpp

Index: Source/core/rendering/RenderRegionSet.cpp
diff --git a/Source/core/rendering/RenderRegionSet.cpp b/Source/core/rendering/RenderRegionSet.cpp
index 933611547d93613fe3d24514e49fe5d97e708d6a..9aaf5edbc06b3b039f093dd01cb356cc37be654a 100644
--- a/Source/core/rendering/RenderRegionSet.cpp
+++ b/Source/core/rendering/RenderRegionSet.cpp
@@ -35,12 +35,6 @@ RenderRegionSet::RenderRegionSet(Element* element, RenderFlowThread* flowThread)
 {
 }
 
-void RenderRegionSet::installFlowThread()
-{
-    // We don't have to do anything, since we were able to connect the flow thread
-    // in the constructor.
-}
-
 void RenderRegionSet::expandToEncompassFlowThreadContentsIfNeeded()
 {
     // Whenever the last region is a set, it always expands its region rect to consume all