Alerting: Remove legacy alerting (#83671)

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

---------

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/pluginsintegration/plugins_integration_test.go

@@ -133,7 +133,6 @@ func verifyCorePluginCatalogue(t *testing.T, ctx context.Context, ps *pluginstor
 	t.Helper()
 
 	expPanels := map[string]struct{}{
-		"alertGroups":    {},
 		"alertlist":      {},
 		"annolist":       {},
 		"barchart":       {},