default-http-backend is no longer being used, can probably be removed

[George Kraft]

This commit in Nov 2018 made it where nginx-ingress-controller is no longer using default-http-backend: https://github.com/charmed-kubernetes/charm-kubernetes-worker/commit/cfc85682dbf11db759b3c34ee7370f83fb42ebca#diff-5bbfdf8fd25bb2cc469d297af201bf86L248

It looks like it's no longer necessary since nginx-ingress-controller 0.20.0+ (which we run on all architectures): https://github.com/kubernetes-retired/contrib/issues/1535#issuecomment-444573061

It should be safe to remove default-http-backend. This will allow us to remove a config option (default-backend-image) and reduce the number of images that need to be made available for offline deployments - and clean up some code, too.