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

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

Index: src/crankshaft/mips64/lithium-codegen-mips64.cc
diff --git a/src/crankshaft/mips64/lithium-codegen-mips64.cc b/src/crankshaft/mips64/lithium-codegen-mips64.cc
index f1b7de5ea3c7d90b30589c93a319f359facbe41b..64c42c69d73069b027d35ee0c9405ecd3ee36ea9 100644
--- a/src/crankshaft/mips64/lithium-codegen-mips64.cc
+++ b/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);