Remove usages of Heap::NewSpaceStart and its external reference

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 3226828c2210436afae35a0440bc489d349aa705..79ef0c03f3af28fbea844d3d0864d09f6a1a541a 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -877,10 +877,6 @@ class Heap {
   // Getters for spaces. =======================================================
   // ===========================================================================
 
-  // Return the starting address and a mask for the new space.  And-masking an
-  // address with the mask will result in the start address of the new space
-  // for all addresses in either semispace.
-  Address NewSpaceStart() { return new_space_.start(); }
   Address NewSpaceTop() { return new_space_.top(); }
 
   NewSpace* new_space() { return &new_space_; }