LibThreading: Clean up pthread mutex attributes and mutex itself

Author: https://github.com/ADKaster
Commit: https://github.com/LadybirdBrowser/ladybird/commit/c19ecf33d97
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3642
Reviewed-by: https://github.com/alimpfard ✅

Libraries/LibThreading/Mutex.h

@@ -27,11 +27,12 @@ public:
         pthread_mutexattr_init(&attr);
         pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
         pthread_mutex_init(&m_mutex, &attr);
+        pthread_mutexattr_destroy(&attr);
     }
     ~Mutex()
     {
         VERIFY(m_lock_count == 0);
-        // FIXME: pthread_mutex_destroy() is not implemented.
+        pthread_mutex_destroy(&m_mutex);
     }
 
     void lock();