remove the outline of the dropdown menu link

remove the outline of the dropdown menu link that is disabled when this with the focus.

Before:
![before](https://f.cloud.github.com/assets/1280255/895605/ddc9de02-facf-11e2-9f10-e80e289f3a8c.jpg)

After:
![after](https://f.cloud.github.com/assets/1280255/895604/ddaacde6-facf-11e2-9b47-06580c3b81d7.jpg)