fix: when upgrading from anonymous, remove correct old user doc

@caolan please review. You could reproduce the problem before
the change with

```js
hoodie.account.anonymousSignUp().done(function(){
  hoodie.account.signUp('test'+Date.now(), 'test');
})
```

You'll see an error in the hoodie server logs. But even
with the fix, you see still see the problem, but the upgrade
works. You see the same error a few times when you run `npm test`.
Could you check that please?