SubmoduleOp: Remove unused RevWalk

Change I021272b7 modified updateSuperProjects to no longer take
a RevWalk. The RevWalk that was being passed into it is no longer
used, but was not removed.

Change-Id: Ieb7d607f06e720b2b0b3fc2779133292a293556b

gerrit-server/src/main/java/com/google/gerrit/server/git/SubmoduleOp.java

@@ -223,7 +223,6 @@ public class SubmoduleOp {
     PersonIdent author = null;
 
     Repository pdb = null;
-    RevWalk recRw = null;
 
     StringBuilder msgbuf = new StringBuilder("Updated git submodules\n\n");
     try {
@@ -343,16 +342,12 @@ public class SubmoduleOp {
         default:
           throw new IOException(rfu.getResult().name());
       }
-      recRw = new RevWalk(pdb);
       // Recursive call: update subscribers of the subscriber
       updateSuperProjects(db, Sets.newHashSet(subscriber));
     } catch (IOException e) {
       throw new SubmoduleException("Cannot update gitlinks for "
           + subscriber.get(), e);
     } finally {
-      if (recRw != null) {
-        recRw.close();
-      }
       if (pdb != null) {
         pdb.close();
       }