UIFe: Removing Gwibber slide from ubiquity-slideshow-ubuntu

[Dylan McCall]

I want to remove the Social slide from ubiquity-slideshow-ubuntu,
because Gwibber is not available in Ubuntu 13.04 and its replacement
(friends-app) is not installed by default. This removes some strings
from our translation template, and it makes the slideshow a little bit
shorter.

UIFe bug report over here:
https://bugs.launchpad.net/ubuntu/+source/ubiquity-slideshow-ubuntu/+bug/1169299

The change is visible in the previewer thing over here:
http://www.dylanmccall.com/ubiquity-slideshow-preview

Thanks!

Dylan McCall