Do not use <abbr> tag

[asavartsov]

Nevermind that using semantic tags as decoration elements isn't a good practice, if some styles globally set for abbr tag, they affects chosen's selection clearing cross. To be specific, Bootstrap 2.0 defines some, so using bootstrap and chosen together means resetting abbr styles specifically for chosen.

[pfiller]

From @mlettini in #1838

Not sure why we are using <abbr>, but it doesn't make sense in this context. We should probably use an <a> for the close button, since it takes an action on click, and that's the tag we use for .search-choice-close on Multi Chosen.