Upgrade VP Backup via Pressable [Backup Stand-alone]: Prevent customers from buying Backup when they already have security bundle

With a Pressable account, customers will have a complimentary Jetpack Security bundle. However, if they install the VaultPress Backup stand-alone plugin, we still allow them to **try** and buy a Backup 10Gb plan instead of showing them the proper Backup in progress UI.

Only at the very end do we tell them Backup is included in the security bundle. We shouldn't allow this to happen, since it's a dead-end. They have to use their back button to get out of this.

- [ ] Remove the flow that takes customers to buy Backup when they already have a Security Bundle. Instead of showing an upsell in the stand-alone UI, the plugin should just show the backups progress UI

Flow visuals & video walkthrough at task 39: p1HpG7-l2l-p2

**This flow is what needs to be removed:**

**Should just be:**