Revert "Revert "HBASE-16698 Performance issue: handlers stuck waiting for CountDownLatch inside WALKey#getWriteEntry under high writing workload; ADDENDUM. Fix findbugs""

This reverts commit f555b5be9c.

A revert of a revert

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/HRegion.java

@@ -3309,7 +3309,6 @@ public class HRegion implements HeapSize, PropagatingConfigurationObserver, Regi
         this.mvcc.advanceTo(batchOp.getReplaySequenceId());
       } else {
         // writeEntry won't be empty if not in replay mode
-        assert writeEntry != null;
         mvcc.completeAndWait(writeEntry);
         writeEntry = null;
       }