TreeWalk.release() only release reader, so remove it

The code releases TreeWalk. This is not nesessary as the TreeWalk.release()
only releases reader object. This is already done in RevWalk.release()
method.

Change-Id: I934ca374461d242ea999ccff8f46f00d6da7c411

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/rest/project/CreateProjectIT.java

@@ -252,7 +252,6 @@ public class CreateProjectIT extends AbstractDaemonTest {
         tw.reset();
       }
     } finally {
-      tw.release();
       rw.release();
       repo.close();
     }

gerrit-server/src/main/java/com/google/gerrit/server/change/GetContent.java

@@ -62,7 +62,6 @@ public class GetContent implements RestReadView<FileResource> {
         if (tw == null) {
           throw new ResourceNotFoundException();
         }
-        try {
         final ObjectLoader object = repo.open(tw.getObjectId(0));
         @SuppressWarnings("resource")
         BinaryResult result = new BinaryResult() {
@@ -72,9 +71,6 @@ public class GetContent implements RestReadView<FileResource> {
           }
         };
         return result.setContentLength(object.getSize()).base64();
-        } finally {
-          tw.release();
-        }
       } finally {
         rw.release();
       }

gerrit-server/src/main/java/com/google/gerrit/server/project/ListDashboards.java

@@ -112,7 +112,6 @@ class ListDashboards implements RestReadView<ProjectResource> {
       throws IOException {
     List<DashboardInfo> list = Lists.newArrayList();
     TreeWalk tw = new TreeWalk(rw.getObjectReader());
-    try {
     tw.addTree(rw.parseTree(ref.getObjectId()));
     tw.setRecursive(true);
     while (tw.next()) {
@@ -133,9 +132,6 @@ class ListDashboards implements RestReadView<ProjectResource> {
         }
       }
     }
-    } finally {
-      tw.release();
-    }
     return list;
   }
 }