MT#62181 core: fix thread exception

AmThread::stop() detaches the thread, which makes it impossible to
::join() it. Only call one or the other.

Change-Id: Icc9301dc9b31856ec7f963835cffd315bbc925bb

core/sems.cpp

@@ -672,7 +672,6 @@ int main(int argc, char* argv[])
   AmPlugIn::dispose();
 
   async_file_writer::instance()->stop();
-  async_file_writer::instance()->join();
 
   unlink(AmConfig::DaemonPidFile.c_str());