libstdc++: Remove TODO comment

We have a comment saying to replace the simple binary_semaphore type
with std::binary_semaphore, which has been done. However, that isn't
defined on all targets. So keep the simple one here that just implements
the parts of the API needed by <stop_token>, and remove the comment
suggesting it should be replaced.

libstdc++-v3/ChangeLog:

	* include/std/stop_token: Remove TODO comment.

libstdc++-v3/include/std/stop_token

@@ -107,7 +107,6 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     }
 
 #ifndef __cpp_lib_semaphore
-    // TODO: replace this with a real implementation of std::binary_semaphore
     struct binary_semaphore
     {
       explicit binary_semaphore(int __d) : _M_counter(__d > 0) { }