os/bluestore: fix improper call of shutdown() to mempool_thread

Because below here we haven't called mempool_thread.init() yet,
and call to mempool_thread.shutdown() will therefore raise an assert instead.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>

src/os/bluestore/BlueStore.cc

@@ -4139,7 +4139,6 @@ int BlueStore::mount()
   return 0;
 
  out_stop:
-  mempool_thread.shutdown();
   _kv_stop();
   wal_wq.drain();
   wal_tp.stop();