remove .sitelink background image

.sitelink contains an image element that outputs the moodlelogo, adding it as a background attribute duplicates the output.
![screen shot 2014-02-28 at 12 57 51 pm](https://f.cloud.github.com/assets/743499/2291129/e6b82da6-a034-11e3-9ae0-25a2a6160eff.png)