Remove empty JS and stylesheet files

The urls.css.scss and static_pages.js.coffee files were just shells of
files that were automatically generated. Since we aren't using them, I'm
removing them to keep things lean. We can always add them back if we
decide that we want them.

I also removed the require_tree directive from the application.js. While
this type of thing is convenient, it makes understanding which assets
you actually need more difficult, so I'd prefer to be explicit. Since it
wasn't including any resources after removing the empty
static_pages.js.coffee, removing this line should have no effect at this
time.

Change-Id: I9259073be0c3619e8daadce61a722f113aaced04