Removed reference to ah-default.css

Hi @nelliemckesson and @adamwitwer,

We don't need to reference ah-default.css directly in any of the CSS stylesheets for PDF for Atlas v2, because the PDFWorker automatically passes the ah-default.css stylesheet located here:

https://github.com/oreillymedia/orm-pdfrenderer/blob/master/ah-default.css

into AntennaHouse. 

I went ahead and removed the reference in atlas_trade_theme in this pull request. If there are any other themes that reference ah-default.css, we should remove those references as well.

Thanks,
Sanders