hurd: Remove some remnants of cthreads

Libc has actually been using mach's lock-internal.h mutex for a long
time already.

mach/lock-intern.h

@@ -95,6 +95,15 @@ __spin_lock_locked (__spin_lock_t *__lock)
 #endif
 
 /* Name space-clean internal interface to mutex locks.  */
+struct mutex {
+	__spin_lock_t __held;
+	__spin_lock_t __lock;
+	const char *__name;
+	void *__head, *__tail;
+	void *__holder;
+};
+
+#define MUTEX_INITIALIZER { __SPIN_LOCK_INITIALIZER }
 
 /* Initialize the newly allocated mutex lock LOCK for further use.  */
 extern void __mutex_init (void *__lock);

mach/mutex-solid.c

@@ -17,11 +17,10 @@
    <https://www.gnu.org/licenses/>.  */
 
 #include <lock-intern.h>
-#include <cthreads.h>
 
-/* If cthreads is linked in, it will define these functions itself to do
-   real cthreads mutex locks.  This file will only be linked in when
-   cthreads is not used, and `mutexes' are in fact just spin locks (and
+/* If pthread is linked in, it will define these functions itself to do
+   real pthread mutex locks.  This file will only be linked in when
+   pthread is not used, and `mutexes' are in fact just spin locks (and
    some unused storage).  */
 
 void