Drop unused Assembler::set_external_target_at() method.

src/mips/assembler-mips.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 519 matching lines @@
   // This sets the branch destination (which gets loaded at the call address).
   // This is for calls and branches within generated code.  The serializer
   // has already deserialized the lui/ori instructions etc.
   inline static void deserialization_set_special_target_at(
       Address instruction_payload, Address target) {
     set_target_address_at(
         instruction_payload - kInstructionsFor32BitConstant * kInstrSize,
         target);
   }
 
-  // This sets the branch destination.
-  // This is for calls and branches to runtime code.
-  inline static void set_external_target_at(Address instruction_payload,
-                                            Address target) {
-    set_target_address_at(instruction_payload, target);
-  }
-
   // Size of an instruction.
   static const int kInstrSize = sizeof(Instr);
 
   // Difference between address of current opcode and target address offset.
   static const int kBranchPCOffset = 4;
 
   // Here we are patching the address in the LUI/ORI instruction pair.
   // These values are used in the serialization process and must be zero for
   // MIPS platform, as Code, Embedded Object or External-reference pointers
   // are split across two consecutive instructions and don't exist separately
@@ skipping 684 matching lines @@
 class EnsureSpace BASE_EMBEDDED {
  public:
   explicit EnsureSpace(Assembler* assembler) {
     assembler->CheckBuffer();
   }
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_ARM_ASSEMBLER_MIPS_H_