doc: Remove an unneeded link to the old ctest repo

(backport <rust-lang#4713>)
(cherry picked from commit 1880ba5)

Author: tgross35

CONTRIBUTING.md

@@ -89,7 +89,7 @@ standard, it's just a list shared among all OSs that declare `#[cfg(unix)]`.
 We have two automated tests running on
 [GitHub Actions](https://github.com/rust-lang/libc/actions):
 
-1. [`libc-test`](https://github.com/gnzlbg/ctest)
+1. `libc-test`
   - `cd libc-test && cargo test`
   - Use the `skip_*()` functions in `build.rs` if you really need a workaround.
 2. Style checker