Remove unused local var. Patch from STL@microsoft.com

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@272622 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/thread/thread.threads/thread.thread.class/thread.thread.member/detach.pass.cpp

@@ -83,7 +83,7 @@ int main()
         assert(!t0.joinable());
         try {
             t0.detach();
-        } catch (std::system_error const& ec) {
+        } catch (std::system_error const&) {
         }
     }
 #endif