remove remnants of critical config
Created by: sqs
The management console (and critical config) was removed in https://github.com/sourcegraph/sourcegraph/pull/7197. Critical config is no longer a concept; it has been merged into site config. This commit updates code and docs to remove remnants of critical config.
@slimsag stated on 2020-02-05 that we should wait until Sourcegraph 3.15 to remove this (https://github.com/sourcegraph/sourcegraph/pull/8284#issuecomment-582381406). It is past that time already. The migration guides referring to critical config remain, of course, so admins of older instances can still find those instructions.