Followup: clean up DuoChat scrolling logic once component supports it natively

Followup from gitlab-ui!4589 (comment 2096418208)

Question: Could you please create a follow-up Issue that we remove: https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/blob/3269f6200dc3821c62a3992b40c971dd9ee55d87/webviews/vue2/gitlab_duo_chat/src/App.vue#L97 Once this is merged and has updated the Gitlab UI dependency so we don't forget about it?

Once the above MR is merged, and VSCode is updated with the latest gitlab/ui, the DuoChat component will handle scrolling to bottom internally. So we can remove the scrolling related code from App.vue