removed live sample iframe border

I liked the idea of having a visual separation to show live samples, but right now this would require more styling imo, this sample image illustrates what it gives on a live sample made this weekend for the mdn weekend Paris
![screen shot 2014-03-10 at 10 30 24](https://f.cloud.github.com/assets/288803/2372209/f3e1bae2-a836-11e3-8f98-c237930aadaa.png)
padding would be nice, but would interfere with layout if the size is 100% (unless we use box-sizing: border-box), but this is only working since ie8.

I think we have a better look without it until we give it a proper appearance