Direct Notification Override Grouping option not working correctly (UI)

**Summary**

When creating an Alert, and configuring direct Notification on it. The UI for Override Grouping does not allow correctly to remove the previous (default) grouping (`grafana_folder`, `alertname`).

When including disabling (...) configuration, the previous grouping is still visible.
And when attempting to remove both labels via backspace, they reapear.

This looks like a frontend (UI) issue that differs a bit to the experience when configuring the same on the Notification Policies page.

In which you can easily remove the previous grouping and override it as desired.

**Grafana Version:** 12.3.* (OSS/Enterprise)
**Grafana Cloud: [tested in Grafana Play](https://play.grafana.org/alerting/grafana/bfanos2mn4e80a/view)**
**Browser** : Chrome Version 144.0.7559.59

No file or test is named. Reproduce the issue in the Alert creation flow by configuring direct Notification and trying to remove the default grafana_folder and alertname grouping labels, then compare it with the Notification Policies page. Done means the labels can be removed and the override grouping remains as configured.

Written by the indexing model from the issue text.