721: [Bug] enableBackup breaks deploy (#793)

(cherry picked from commit 288bab6)

Author: sevu1

lib/backup.ts

@@ -39,10 +39,6 @@ export default class Backup extends Stack {
                     scheduleExpression: Schedule.cron({
                         minute: '0',
                         hour: '5',
-                        day: '?',
-                        month: '*',
-                        weekDay: '*',
-                        year: '*',
                     }),
                 }),
             ],
@@ -57,8 +53,8 @@ export default class Backup extends Stack {
                         statements: [
                             new PolicyStatement({
                                 effect: Effect.ALLOW,
-                                principals: [new ServicePrincipal('backup.amazonaws.com')],
                                 actions: ['sts:AssumeRole'],
+                                resources: ['*'],
                             }),
                         ],
                     }),