Allow account deletion even when email authentication is disabled

### Describe the problem

Currently, if a Weblate instance has email authentication disabled (`social_core.backends.email.EmailAuth` is not defined in `AUTHENTICATION_BACKENDS`, and a different backend - e.g. a OAuth2 provider - is used), the delete user page returns a 404.

### Describe the solution you would like

The user would be able to delete their account even if they have no password set (for example, they could be asked to verify the request by logging in with whatever method they used to log in for this session, or the deletion page could skip the verification entirely and simply delete the user's account).

### Describe alternatives you have considered

Admins can still delete users manually upon request, but this is impractical.