Cleanup kv_store alertmanager namespace when disabling unified alerting (#39554) (#39576)

(cherry picked from commit ba4242bdb9)

Co-authored-by: Sofia Papagiannaki <papagian@users.noreply.github.com>

pkg/services/sqlstore/migrations/ualert/ualert.go

@@ -19,6 +19,8 @@ const DASHBOARD_FOLDER = "Migrated %s"
 // during alert migration cleanup.
 const FOLDER_CREATED_BY = -8
 
+const KV_NAMESPACE = "alertmanager"
+
 var migTitle = "move dashboard alerts to unified alerting"
 
 var rmMigTitle = "remove unified alerting data"
@@ -417,6 +419,18 @@ func (m *rmMigration) Exec(sess *xorm.Session, mg *migrator.Migrator) error {
 		return err
 	}
 
+	exists, err := sess.IsTableExist("kv_store")
+	if err != nil {
+		return err
+	}
+
+	if exists {
+		_, err = sess.Exec("delete from kv_store where namespace = ?", KV_NAMESPACE)
+		if err != nil {
+			return err
+		}
+	}
+
 	files, err := getSilenceFileNamesForAllOrgs(mg)
 	if err != nil {
 		return err