removes squadcast from docs (#23213)

removes squadcast since its not a core alert notifier. Mentioning all
services that support the Grafana webhook is not feasible for our docs

docs/sources/alerting/notifications.md

@@ -203,7 +203,6 @@ Prometheus Alertmanager | `prometheus-alertmanager` | yes, external only | yes
 Pushover | `pushover` | yes | no
 Sensu | `sensu` | yes, external only | no
 Slack | `slack` | yes | no
-Squadcast | `webhook` | no | no
 Telegram | `telegram` | yes | no
 Threema | `threema` | yes, external only | no
 VictorOps | `victorops` | yes, external only | no