Getting push notifications for deleted accounts

Steps to reproduce

1. setup two accounts (in my case from two different Nextclouds) in the iOS app
2. delete one of them from the iOS app
3. create a notification on the server (for testing admin_notification app is the easiest) for this deleted account

Expected behaviour

There is no push notification on the device, because the account was removed.

Actual behaviour

A notification with "Nextcloud notification" as text is shown on the device.

Screenshots

Reasoning or why should it be changed/implemented?

I remember that we had discussions in the past that there was a "Push notification could not be decrypted" message. This seems to be the same but with different text. But there are two things to fix:

• if there comes a notification that can't be decrypted: don't show it as it has no value at all
• try to avoid the push in the first place: on account deletion the push registration should be removed as well

cc @marinofaggiana