MDEV-15795 Stack exceeded if pthread_attr_setstacksize(&thr_attr,8196) succeeds

on Linux this pthread_attr_setstacksize() fails with EINVAL
"The stack size is less than PTHREAD_STACK_MIN (16384) bytes".

But on FreeBSD it succeeds and causes a crash later, as 8196 is too little.

Let's keep the stack at its default size in the timer thread.

mysys/my_pthread.c

@@ -263,7 +263,6 @@ int sigwait(sigset_t *setp, int *sigp)
     pthread_attr_init(&thr_attr);
     pthread_attr_setscope(&thr_attr,PTHREAD_SCOPE_PROCESS);
     pthread_attr_setdetachstate(&thr_attr,PTHREAD_CREATE_DETACHED);
-    pthread_attr_setstacksize(&thr_attr,8196);
     pthread_create(&sigwait_thread_id, &thr_attr, sigwait_thread, setp);
     pthread_attr_destroy(&thr_attr);
   }

mysys/thr_alarm.c

@@ -114,7 +114,6 @@ void init_thr_alarm(uint max_alarms)
     pthread_attr_init(&thr_attr);
     pthread_attr_setscope(&thr_attr,PTHREAD_SCOPE_PROCESS);
     pthread_attr_setdetachstate(&thr_attr,PTHREAD_CREATE_DETACHED);
-    pthread_attr_setstacksize(&thr_attr,8196);
     mysql_thread_create(key_thread_alarm,
                         &alarm_thread, &thr_attr, alarm_handler, NULL);
     pthread_attr_destroy(&thr_attr);

mysys/thr_timer.c

@@ -85,7 +85,6 @@ my_bool init_thr_timer(uint alloc_timers)
   /* Create a thread to handle timers */
   pthread_attr_init(&thr_attr);
   pthread_attr_setscope(&thr_attr,PTHREAD_SCOPE_PROCESS);
-  pthread_attr_setstacksize(&thr_attr,8196);
   thr_timer_inited= 1;
   if (mysql_thread_create(key_thread_timer, &timer_thread, &thr_attr,
                           timer_handler, NULL))