Remove the enableGhecDr flag

[robertbrignull]

The feature is now fully enabled on the API side for all GHEC-DR instances. We should remove the feature flag on the extension side too. To use it now all you'll need to do is set github-enterprise.uri.

Checklist

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.