Remove settings usernames from the code

[saledjenic]

Due to changes made on the status-go side, where settings.Usernames property is remove from the settings we need to clean the desktop app code since it's not in use anymore.

Related status-go PR: status-im/status-go#3964

[igor-sirotin]

Actually it should be fine already, though I didn't check the code.
I switched the desktop to the new DB table (I actually created it), so it shouldn't be using the old one.

Maybe just some unused variables/methods pointing to the settings.Usernames.

[saledjenic]

Yes, we just need to go over that part and remove unused code.