Remove the obsolete OverwriteMode optimization.

src/full-codegen.h

Index: src/full-codegen.h
diff --git a/src/full-codegen.h b/src/full-codegen.h
index ce691c37e19f88806e3b2c72db419f4b51535c9a..9a2e7460fc3338ae89087f6e5f0f22bce76c7e4e 100644
--- a/src/full-codegen.h
+++ b/src/full-codegen.h
@@ -584,15 +584,12 @@ class FullCodeGenerator: public AstVisitor {
 
   // Apply the compound assignment operator. Expects the left operand on top
   // of the stack and the right one in the accumulator.
-  void EmitBinaryOp(BinaryOperation* expr,
-                    Token::Value op,
-                    OverwriteMode mode);
+  void EmitBinaryOp(BinaryOperation* expr, Token::Value op);
 
   // Helper functions for generating inlined smi code for certain
   // binary operations.
   void EmitInlineSmiBinaryOp(BinaryOperation* expr,
                              Token::Value op,
-                             OverwriteMode mode,
                              Expression* left,
                              Expression* right);