Remove asset paths property from origami.json

We added this a few months ago with the thinking that modules may want to express which paths within the repo that they are likely to make use of as static assets, so that product developers can make only those assets available publicly.  However:

* [No module](https://github.com/search?l=&q=assetpaths+user%3Afinancial-times&ref=advsearch&type=Code&utf8=%E2%9C%93) has actually used this feature.
* It's fragile - you have to remember to update the assetpaths when you add a new asset reference to CSS or JS
* Identifying static assets in CSS can be done more reliably by analysing compiled bundles for which there are [trivial solutions available](https://gist.github.com/parkerl/1771976).
* It adds complexity to Origami, which has a cost

I propose to kill it.  All in favour say merge.