Remove custom JS for demo site

Pull request #1199 highlighted that there's a copy of ui.js used for the
demo site. At d98f3446356ef290529671621db843263c2dc5f7, when that copy
was last modified, only one line was different between the
sites/demo/static/demo/js/oscar/ui.js and
oscar/static/oscar/js/oscar/ui.js.

Similarly, there was a copy of dashboard.js which wasn't even used and
which was no different from the stock one.

As I've not spotted any regressions when browsing the demo site, I've
reverted to using the stock versions and removed the copies.

Fixes #1199