gthread-posix: Do not do unguarded and non-atomic assignment of an atomic

We ended up always skipping showing the scheduler settings errors after
the first call, while we were already setting such variable atomically
in case it needed to.

Related to: #1672

glib/gthread-posix.c

@@ -1265,7 +1265,6 @@ linux_pthread_proxy (void *data)
         g_critical ("Failed to set scheduler settings: %s", g_strerror (errsv));
       else if (res == -1)
         g_debug ("Failed to set scheduler settings: %s", g_strerror (errsv));
-      printed_scheduler_warning = TRUE;
     }
 
   return thread->proxy (data);