Alerting: Remove legacy alerting (#83671)

Removes legacy alerting, so long and thanks for all the fish! 🐟

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Co-authored-by: Matthew Jacobson <matthew.jacobson@grafana.com>
Co-authored-by: Sonia Aguilar <soniaAguilarPeiron@users.noreply.github.com>
Co-authored-by: Armand Grillet <armandgrillet@users.noreply.github.com>
Co-authored-by: William Wernert <rwwiv@users.noreply.github.com>
Co-authored-by: Yuri Tseretyan <yuriy.tseretyan@grafana.com>

pkg/services/navtree/models.go

@@ -39,7 +39,6 @@ const (
 	NavIDAlertsAndIncidents   = "alerts-and-incidents"
 	NavIDTestingAndSynthetics = "testing-and-synthetics"
 	NavIDAlerting             = "alerting"
-	NavIDAlertingLegacy       = "alerting-legacy"
 	NavIDMonitoring           = "monitoring"
 	NavIDInfrastructure       = "infrastructure"
 	NavIDFrontend             = "frontend"