chore: Remove googleapis#2159 from backlog as it has been implemented…

… in googleapis#2394.

BACKLOG.md

@@ -46,6 +46,5 @@ the request and want to put work into it.
 - Issue [#1812 Support for limited input device flow](https://github.com/googleapis/google-api-dotnet-client/issues/1812)
 - Issue [#1827 PCKE flow should no require client_secret](https://github.com/googleapis/google-api-dotnet-client/issues/1827)
 - Issue [#2011 Add support for Domain-Wide Delegation using ImpersonatedCredential](https://github.com/googleapis/google-api-dotnet-client/issues/2011)
-- Issue [#2159 Impersonation for gcloud credentials](https://github.com/googleapis/google-api-dotnet-client/issues/2159)
 - Status: ongoing internal effort to consolidate and standardize OAuth features across language libraries. These feature requests are on the Auth team backlog.
 - Action: Auth team will prioritize these issues and we'll work on them accordingly.