Remove breadcrumb link to contents on contents page

It’s confusing for users if we have a breadcrumb linking to the page they are currently on.
This PR removes the link when a user is on the Contents page. As shown:

![no-link](https://cloud.githubusercontent.com/assets/265403/4662912/bbb9b2ac-5536-11e4-94a4-fab12d7b8532.png)