Remove an unused variable in a test of std::c_str.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>

src/libstd/c_str.rs

@@ -524,7 +524,6 @@ mod tests {
     #[test]
     fn test_to_c_str_fail() {
         use task;
-        let mut error_happened = false;
         assert!(task::try(proc() { "he\x00llo".to_c_str() }).is_err());
     }