remove experimentalFeatures.configVars
Created by: sqs
fix https://github.com/sourcegraph/enterprise/issues/12547
This was never enabled by default and was not widely used or needed. It will also become irrelevant when we change the way that site config is managed (from being an external file to being managed by Sourcegraph itself) in https://github.com/sourcegraph/about/pull/36.
This PR updates the CHANGELOG.md file to describe any user-facing changes.