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

TBR=bmeurer@chromium.org
NOTRY=true
NOTREECHECKS=true

Review-Url: https://codereview.chromium.org/2621123002
Cr-Commit-Position: refs/heads/master@{#42198}

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

@@ -2490,7 +2490,6 @@ void LCodeGen::DoLoadContextSlot(LLoadContextSlot* instr) {
 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);

src/crankshaft/mips64/lithium-codegen-mips64.cc

@@ -2613,7 +2613,6 @@ void LCodeGen::DoLoadContextSlot(LLoadContextSlot* instr) {
 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());
 
   __ sd(value, target);