remove the cacheSeconds/maxAge param

[chris48s]

In some previous discussions, we've talked about maybe removing the ability for the user to manually pass a cacheSeconds/maxAge param once we've got rid of all the legacy services:

#2205 (comment)
#2755 (comment)

This param was introduced before we were using a CDN to cache at all and works around a performance problem which we've mostly now fixed. Now that we don't have any legacy services left every badge should have a sensible downstream cache length defined (based on the badge category: build, version, license, etc). I think now it is reasonable to retire that feature and simplify our internals a bit.

I don't mind taking this on, but its a bit of a fiddly job, so I'd like to make sure we're agreed that is a sensible thing to do before I start on it.