[Feature flag] Enable and remove `sourcegraph` feature flag
## What
Remove the `sourcegraph` feature flag ...
## Owners
- Team: ~"group::editor"
- Most appropriate slack channel to reach out to: `#g_create_editor`
- Best individual to reach out to: @pslaughter
## Expectations
### What are we expecting to happen?
- Not much for `.com`.
- For self-hosted users, they'll have the ability to enable a Sourcegraph integration for their instance.
### What might happen if this goes wrong?
- If this feature flag is removed, we will not be able to disable Sourcegraph other than through the admin settings.
### What can we monitor to detect problems with this?
- Client-side errors are sent to Sentry.
## Beta groups/projects
- `gitlab-org/gitlab` project
- `gitlab-org`/`gitlab-com` groups
- `sourcegraph` group
## Roll Out Steps
- [x] Confirm that QA tests pass with the feature flag enabled (if you're unsure how, contact the relevant [stable counterpart in the Quality department](https://about.gitlab.com/handbook/engineering/quality/#individual-contributors))
- [x] Enable on staging (`/chatops run feature set feature_name true --staging`)
- [x] Test on staging
- [x] Ensure that documentation has been updated
- [x] Enable on GitLab.com for individual groups/projects listed above and verify behaviour (`/chatops run feature set --project=gitlab-org/gitlab feature_name true`)
- [x] Coordinate a time to enable the flag with the SRE oncall and release managers
- In `#production` mention `@sre-oncall` and `@release-managers`. Once an SRE on call and Release Manager on call confirm, you can proceed with the rollout
- [x] Announce on the issue an estimated time this will be enabled on GitLab.com
- [x] Enable on GitLab.com by running chatops command in `#production` (`/chatops run feature set feature_name true`)
- [x] Cross post chatops Slack command to `#support_gitlab-com` ([more guidance when this is necessary in the dev docs](https://docs.gitlab.com/ee/development/feature_flags/controls.html#where-to-run-commands)) and in your team channel
- [x] Announce on the issue that the flag has been enabled
- [ ] Remove feature flag and add changelog entry
- [ ] After the flag removal is deployed, [clean up the feature flag](https://docs.gitlab.com/ee/development/feature_flags/controls.html#cleaning-up) by running chatops command in `#production` channel
## Rollback Steps
- [ ] This feature can be disabled by running the following Chatops command:
```
/chatops run feature set --project=gitlab-org/gitlab feature_name false
```
### Availability and Testing
Please coordinate the changes to the feature flag status with a SET so that QA team can monitor for any failures.
issue