remove old debugging message (#487)

It doesn't seem this is needed.

util/env_posix.cc

@@ -1635,10 +1635,6 @@ class PosixEnv : public Env {
             WakeUpAllThreads();
           }
           PthreadCall("unlock", pthread_mutex_unlock(&mu_));
-          // TODO(sdong): temp logging. Need to help debugging. Remove it when
-          // the feature is proved to be stable.
-          fprintf(stdout, "Bg thread %zu terminates %llx\n", thread_id,
-                  static_cast<long long unsigned int>(gettid()));
           break;
         }
         void (*function)(void*) = queue_.front().function;