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

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

Index: src/crankshaft/mips/lithium-codegen-mips.cc
diff --git a/src/crankshaft/mips/lithium-codegen-mips.cc b/src/crankshaft/mips/lithium-codegen-mips.cc
index c295ae17f09a03f28111fc87c1181187167b356e..68dcb21dd72b3bd6031d93c8712b82b5548f34be 100644
--- a/src/crankshaft/mips/lithium-codegen-mips.cc
+++ b/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);