Remove experimental ExternalSymbolCallback feature. This is not needed...

src/heap.h

Index: src/heap.h
===================================================================
--- src/heap.h	(revision 1252)
+++ src/heap.h	(working copy)
@@ -544,8 +544,6 @@
       ExternalAsciiString::Resource* resource);
   static Object* AllocateExternalStringFromTwoByte(
       ExternalTwoByteString::Resource* resource);
-  static Object* AllocateExternalSymbolFromTwoByte(
-      ExternalTwoByteString::Resource* resource);
 
   // Allocates an uninitialized object.  The memory is non-executable if the
   // hardware and OS allow.
@@ -624,10 +622,6 @@
     global_gc_epilogue_callback_ = callback;
   }
 
-  static void SetExternalSymbolCallback(ExternalSymbolCallback callback) {
-    global_external_symbol_callback_ = callback;
-  }
-
   // Heap roots
 #define ROOT_ACCESSOR(type, name) static type* name() { return name##_; }
   ROOT_LIST(ROOT_ACCESSOR)
@@ -887,9 +881,6 @@
   static GCCallback global_gc_prologue_callback_;
   static GCCallback global_gc_epilogue_callback_;
 
-  // Callback function used for allocating external symbols.
-  static ExternalSymbolCallback global_external_symbol_callback_;
-
   // Checks whether a global GC is necessary
   static GarbageCollector SelectGarbageCollector(AllocationSpace space);