[Don't merge yet] Remove mainstream browse from Frontend

https://www.pivotaltracker.com/story/show/72442394

Soon, mainstream browse will be served from the [collections app](https://github.com/alphagov/collections) and we will be removing mainstream browse 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).

Whilst review on this would be appreciated, this should not be merged until after mainstream browse in collections has gone live.