Remove selection overrides

I noticed this while testing #1824. Having the browser text selection behaviour change when it moves over a highlight is extremely confusing. 

This PR...

* removes that behaviour
* the highlight mixin as it now does nothing.
* brings the PDFjs override file inline with common.css

**before**
![broken](https://cloud.githubusercontent.com/assets/47144/5525635/63ac06b8-89e9-11e4-997c-5aa66a441e50.gif)

**after**
![fixed](https://cloud.githubusercontent.com/assets/47144/5525634/637f1392-89e9-11e4-8964-cf19d733c64a.gif)