Remove mainstream browse from Frontend

https://www.pivotaltracker.com/story/show/72442394

Browse is now served from the [collections app](https://github.com/alphagov/collections), so we should remove the old browse behaviour from Frontend. 

This removes code for mainstream browse, along with its tests, routes and assets. The redirect routes removed in this pull request have [already been added to router-data](https://github.gds/gds/router-data/pull/100).