Get tests compiling with -Wunused-local-typedef

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@346914 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/thread/thread.threads/thread.thread.this/sleep_for.pass.cpp

@@ -57,7 +57,6 @@ int main()
 
     typedef std::chrono::system_clock Clock;
     typedef Clock::time_point time_point;
-    typedef Clock::duration duration;
     std::chrono::milliseconds ms(500);
     time_point t0 = Clock::now();
     std::this_thread::sleep_for(ms);