Documentation: undocument gc'd function graph_release()

graph_release() was removed in 064bfbd.  Cut it from the API
documentation and a comment.

Signed-off-by: Greg Price <price@ksplice.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-history-graph.txt

@@ -11,9 +11,6 @@ Core functions:
 
 * `graph_init()` creates a new `struct git_graph`
 
-* `graph_release()` destroys a `struct git_graph`, and frees the memory
-  associated with it.
-
 * `graph_update()` moves the graph to a new commit.
 
 * `graph_next_line()` outputs the next line of the graph into a strbuf.  It
@@ -134,8 +131,6 @@ while ((commit = get_revision(opts)) != NULL) {
 			putchar(opts->diffopt.line_termination);
 	}
 }
-
-graph_release(graph);
 ------------
 
 Sample output

graph.h

@@ -6,7 +6,6 @@ struct git_graph;
 
 /*
  * Create a new struct git_graph.
- * The graph should be freed with graph_release() when no longer needed.
  */
 struct git_graph *graph_init(struct rev_info *opt);