ART: Remove unused variables.

Curious that neither Clang nor GCC flagged those.

Change-Id: Ie1866d8171651c768e378d829a669cc57cab1f09
diff --git a/runtime/arch/mips64/instruction_set_features_mips64.cc b/runtime/arch/mips64/instruction_set_features_mips64.cc
index 8c48a08..5c0c914 100644
--- a/runtime/arch/mips64/instruction_set_features_mips64.cc
+++ b/runtime/arch/mips64/instruction_set_features_mips64.cc
@@ -27,7 +27,6 @@
 const Mips64InstructionSetFeatures* Mips64InstructionSetFeatures::FromVariant(
     const std::string& variant, std::string* error_msg ATTRIBUTE_UNUSED) {
   if (variant != "default" && variant != "mips64r6") {
-    std::ostringstream os;
     LOG(WARNING) << "Unexpected CPU variant for Mips64 using defaults: " << variant;
   }
   bool smp = true;  // Conservative default.
diff --git a/runtime/arch/x86/instruction_set_features_x86.cc b/runtime/arch/x86/instruction_set_features_x86.cc
index 8227633..ef39999 100644
--- a/runtime/arch/x86/instruction_set_features_x86.cc
+++ b/runtime/arch/x86/instruction_set_features_x86.cc
@@ -63,7 +63,6 @@
   bool known_variant = FindVariantInArray(x86_known_variants, arraysize(x86_known_variants),
                                           variant);
   if (!known_variant && variant != "default") {
-    std::ostringstream os;
     LOG(WARNING) << "Unexpected CPU variant for X86 using defaults: " << variant;
   }