MDEV-35574 remove obsolete pthread_exit calls

Threads can normally exit without a explicit pthread_exit call.

There seem to date to old glibc bugs, many around 2.2.5.

The semi related bug was https://bugs.mysql.com/bug.php?id=82886.

To improve safety in the signal handlers DBUG_* code was removed.

These where also needed to avoid some MSAN unresolved stack issues.

This is effectively a backport of 2719cc4925.

unittest/sql/my_apc-t.cc

@@ -108,7 +108,6 @@ void *test_apc_service_thread(void *ptr)
   apc_target.destroy();
   mysql_mutex_destroy(&target_mutex);
   my_thread_end();
-  pthread_exit(0);
   return NULL;
 }