Remove unused metrics and update code suggestions to user count in ai_impact_over_time.js
## What needs to be done
Remove unused metrics and their associated visualizations from `ee/app/assets/javascripts/analytics/analytics_dashboards/data_sources/ai_impact_over_time.js`.
The following metrics and visualizations should be deleted:
- `AI_METRICS.DUO_CHAT_USAGE_RATE` (line 52)
- `AI_METRICS.DUO_USAGE_RATE` (line 63)
Update the following metric and visualization:
- `AI_METRICS.CODE_SUGGESTIONS_USAGE_RATE` - rename to `AI_METRICS.CODE_SUGGESTIONS_USERS` and update to display absolute user count instead of rate
Note: The code suggestions users tile will later be replaced in [#587299](https://gitlab.com/gitlab-org/gitlab/-/work_items/587299).
## Blocking
[#590326](https://gitlab.com/gitlab-org/gitlab/-/work_items/590326)
issue