[webhook] Don't depend on system.Namespace

> I am super unthrilled to have this code depend on system.Namespace, but that should work :)
But also we can only use it for the system namespace, I can't limit it to any other (which we might want at some point, maybe).

_Originally posted by @vagababov in https://github.com/knative/pkg/pull/1158_

Start by locating the webhook code that depends on system.Namespace and reviewing how that dependency limits namespaces beyond the system namespace. Determine the intended replacement from the surrounding webhook implementation and discussion; done means the webhook no longer has this dependency while preserving its current behavior.

Written by the indexing model from the issue text.