Remove build badge path from routes

Older pipeline badges come from the time we still considered everything builds, and the badges for these perform a GET request on build.svg. This is being moved to pipeline.svg with gitlab-ce!12966

The old paths are redirected to the new one, with a 302 status code. However, people will need to manually update their badges, and therefor we need to support this route a long time. I've scheduled this issue for gitlab-ce%"10.0" now, but this is just a way of saying; we can't drop it before than, we might just want to keep supporting the older route.