Remove unnecessary std::move from variant_map.h

This triggers Clang's pessimizing-move warning.

Change-Id: Icb33643f28d1d159a87a17235c4d210209199dac
diff --git a/runtime/base/variant_map.h b/runtime/base/variant_map.h
index 1d7596a..82e5d2e 100644
--- a/runtime/base/variant_map.h
+++ b/runtime/base/variant_map.h
@@ -257,8 +257,7 @@
     if (ptr != nullptr) {
       return std::move(*ptr);
     } else {
-      TValue default_value = key.CreateDefaultValue();
-      return std::move(default_value);
+      return key.CreateDefaultValue();
     }
   }