Revert "Replace HeapNumber as doublebox with an explicit MutableHeapNumber."

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index 7a27776a5e04dfeaa5767ba8d90b9f34cbaf8c35..8a0ffa61540b44bb4cf3dd55891049c4f0b56faa 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -1186,8 +1186,7 @@ class MacroAssembler: public Assembler {
   // space is full.
   void AllocateHeapNumber(Register result,
                           Register scratch,
-                          Label* gc_required,
-                          MutableMode mode = IMMUTABLE);
+                          Label* gc_required);
 
   // Allocate a sequential string. All the header fields of the string object
   // are initialized.