abbr[title] should not have border-bottom

Created by: Rewt0r

On the latest Firefox releases this is showing 2 underlines on abbr elements, the specification states that the default CSS for abbr[title] should be:

abbr[title], acronym[title] { text-decoration: dotted underline; }

Therefore this is not required in Bootstrap.

Refer to: http://www.w3.org/TR/html5/rendering.html#phrasing-content-0