Remove unused repository field from RevWalk

The comment about legacy Tag and Object types no longer applies,
though prior to Idb273d5a92849b42935ac14eed73b796b80aad50 the field
was still being used by RewriteTreeFilter.

Change-Id: I9ee5da8f8a3b61c9cf543817c03117ee0609dd8f

org.eclipse.jgit.test/tst/org/eclipse/jgit/revwalk/RevWalkShallowTest.java

 		final StringBuilder builder = new StringBuilder();
 		for (ObjectId commit : shallowCommits)
 			builder.append(commit.getName() + "\n");
-		JGitTestUtil.write(new File(rw.repository.getDirectory(), "shallow"),
+		JGitTestUtil.write(new File(db.getDirectory(), "shallow"),
 				builder.toString());
 	}
 }

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/RevWalk.java

 
 	private static final int APP_FLAGS = -1 & ~((1 << RESERVED_FLAGS) - 1);
 
-	/** Exists <b>ONLY</b> to support legacy Tag and Commit objects. */
-	final Repository repository;
-
 	final ObjectReader reader;
 
 	final MutableObjectId idBuffer;
 	 *            released by the caller.
 	 */
 	public RevWalk(final Repository repo) {
-		this(repo, repo.newObjectReader());
+		this(repo.newObjectReader());
 	}
 
 	/**
 	 *            required.
 	 */
 	public RevWalk(ObjectReader or) {
-		this(null, or);
-	}
-
-	private RevWalk(final Repository repo, final ObjectReader or) {
-		repository = repo;
 		reader = or;
 		idBuffer = new MutableObjectId();
 		objects = new ObjectIdOwnerMap<RevObject>();