Remove the extra horizontal line on the edit single mapping page

- An extra horizontal line appeared when "archive" was selected as the
  mapping type.

Before:

![screen shot 2014-10-24 at 20 17 48](https://cloud.githubusercontent.com/assets/355033/4791443/34d0ad22-5ddd-11e4-956e-72d63f2ed3a9.png)

After:

![screen shot 2014-10-27 at 13 29 51](https://cloud.githubusercontent.com/assets/355033/4791465/5d555284-5ddd-11e4-9d96-1881ef5e3cdc.png)