Drop unused Assembler::set_external_target_at() method.

src/x64/assembler-x64.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // All Rights Reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 // - Redistributions of source code must retain the above copyright notice,
 // this list of conditions and the following disclaimer.
 //
@@ skipping 568 matching lines @@
   // of that call in the instruction stream.
   static inline Address target_address_from_return_address(Address pc);
 
   // This sets the branch destination (which is in the instruction on x64).
   // This is for calls and branches within generated code.
   inline static void deserialization_set_special_target_at(
       Address instruction_payload, Address target) {
     set_target_address_at(instruction_payload, target);
   }
 
-  // This sets the branch destination (which is a load instruction on x64).
-  // This is for calls and branches to runtime code.
-  inline static void set_external_target_at(Address instruction_payload,
-                                            Address target) {
-    *reinterpret_cast<Address*>(instruction_payload) = target;
-  }
-
   inline Handle<Object> code_target_object_handle_at(Address pc);
   inline Address runtime_entry_at(Address pc);
   // Number of bytes taken up by the branch target in the code.
   static const int kSpecialTargetSize = 4;  // Use 32-bit displacement.
   // Distance between the address of the code target in the call instruction
   // and the return address pushed on the stack.
   static const int kCallTargetAddressOffset = 4;  // Use 32-bit displacement.
   // The length of call(kScratchRegister).
   static const int kCallScratchRegisterInstructionLength = 3;
   // The length of call(Immediate32).
@@ skipping 1113 matching lines @@
  private:
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_ASSEMBLER_X64_H_