Removed unreachable code

* node.c (rb_ast_dispose): since `ast->node_buffer` is freed in
  `rb_ast_free()`, it should be always NULL.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65031 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

node.c

@@ -1139,7 +1139,6 @@ void
 rb_ast_dispose(rb_ast_t *ast)
 {
     rb_ast_free(ast);
-    if (ast->node_buffer) RB_OBJ_WRITE(ast, &ast->node_buffer->mark_ary, Qnil);
 }
 
 void