For now do not guard against already failed engine

Author: jasontedor

core/src/main/java/org/elasticsearch/index/shard/IndexShard.java

@@ -759,13 +759,7 @@ public void releaseSnapshot(IndexCommit snapshot) throws IOException {
      */
     public void failShard(String reason, @Nullable Throwable e) {
         // fail the engine. This will cause this shard to also be removed from the node's index service.
-        final Engine engine = getEngineOrNull();
-        if (engine == null) {
-            logger.trace("ignoring request to fail the shard, we're already closed. (reason: [{}])", e, reason);
-
-        } else {
-            engine.failEngine(reason, e);
-        }
+        getEngine().failEngine(reason, e);
     }
 
     public Engine.Searcher acquireSearcher(String source) {