[libcxx] [test] Fix Clang -Wunused-local-typedef warnings.

Fix D34536.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@308534 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/thread/thread.threads/thread.thread.this/sleep_until.pass.cpp

@@ -22,7 +22,6 @@ 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_until(t0 + ms);