[crankshaft] Fix mips/mips64 build: remove unused variable

src/crankshaft/mips/lithium-codegen-mips.cc

 // Copyright 2012 the V8 project authors. All rights reserved.7
 // 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.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 2472 matching lines @@
   Register context = ToRegister(instr->context());
   Register result = ToRegister(instr->result());
 
   __ lw(result, ContextMemOperand(context, instr->slot_index()));
 }
 
 
 void LCodeGen::DoStoreContextSlot(LStoreContextSlot* instr) {
   Register context = ToRegister(instr->context());
   Register value = ToRegister(instr->value());
-  Register scratch = scratch0();
   MemOperand target = ContextMemOperand(context, instr->slot_index());
 
   __ sw(value, target);
   if (instr->hydrogen()->NeedsWriteBarrier()) {
     SmiCheck check_needed =
         instr->hydrogen()->value()->type().IsHeapObject()
             ? OMIT_SMI_CHECK : INLINE_SMI_CHECK;
     __ RecordWriteContextSlot(context,
                               target.offset(),
                               value,
@@ skipping 2887 matching lines @@
   __ lw(result, FieldMemOperand(scratch,
                                 FixedArray::kHeaderSize - kPointerSize));
   __ bind(deferred->exit());
   __ bind(&done);
 }
 
 #undef __
 
 }  // namespace internal
 }  // namespace v8