os/bluestore/BlueStore: optimize WALWQ::flush()

In fact ThreadPool::drain will do
a)wait WorkQueue::empty() is true
b)wait processing thread to complete.
So it can remove the same work which wait WorkQueue::empty().

Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>

src/os/bluestore/BlueStore.h

@@ -749,11 +749,6 @@ public:
     }
 
     void flush() {
-      lock();
-      while (!wal_queue.empty()) {
-	_wait();
-      }
-      unlock();
       drain();
     }
   };