When a user deletes account, authoritative keyserver sends updates to known peers, which in user-reducer are processed and the user is deleted from their userStores.
We want the same for auxUserStore, when a peer deletes their account.

When relying on authoritative keyserver, we can simply copy the action to auxUserStore.
Scenario with relyingOnAuthoritativeKeyserve = false is handled in the next diff

Depends on D12945

• Register two users and make sure they know about each other (have entries in user stores)
• Delete account for one user
• Make sure it gets removed from the other user's userStore and auxUserStore