struct tree: remove unused field "parent"

The field is not used anymore, after the recent ls-tree rewrite.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

tree.c

@@ -200,7 +200,6 @@ int parse_tree_buffer(struct tree *item, void *buffer, unsigned long size)
 		}
 		if (obj)
 			n_refs++;
-		entry->parent = NULL; /* needs to be filled by the user */
 		*list_p = entry;
 		list_p = &entry->next;
 	}

tree.h

@@ -18,7 +18,6 @@ struct tree_entry_list {
 		struct tree *tree;
 		struct blob *blob;
 	} item;
-	struct tree_entry_list *parent;
 };
 
 struct tree {