pthread destroy: revert. No way to please static analysis.

src/common/dtpthread.h

@@ -64,8 +64,6 @@ typedef struct dt_pthread_rwlock_t
 
 static inline int dt_pthread_mutex_destroy(dt_pthread_mutex_t *mutex)
 {
-  // Prevent mutexes from being destroyed if they are still locked by a thread.
-  pthread_mutex_unlock(&mutex->mutex);
   const int ret = pthread_mutex_destroy(&(mutex->mutex));
   assert(!ret);
 
@@ -323,8 +321,6 @@ static inline int dt_pthread_mutex_unlock(dt_pthread_mutex_t *mutex) RELEASE(mut
 
 static inline int dt_pthread_mutex_destroy(dt_pthread_mutex_t *mutex)
 {
-  // Prevent mutexes from being destroyed if they are still locked by a thread.
-  pthread_mutex_unlock(&mutex->mutex);
   return pthread_mutex_destroy(&mutex->mutex);
 };