feat: remove default extension registry
Created by: chrismwendt
Here's what the OSS extension registry looks like by default:
@sqs Could you make a pass on the wording/CTA for the empty registry? Will users be expecting more information here about what extensions are and which license they need in order to get started with a private extension registry?
Resolves https://github.com/sourcegraph/enterprise/issues/13550
This PR does not need to update the CHANGELOG because ...