Remove unneeded, outdated platforms and applications code (bug 1022830)

https://bugzilla.mozilla.org/show_bug.cgi?id=1022830

Tests are green https://ci-addons.allizom.org/job/marketplace-build-branch/92/
yeah, because `amo.ext.cache` is only a wrapper around `caching.ext.FragmentCacheExtension` with `request.APP` included in the key. Since `request.APP` is going away the wrapper should go.