Drop some backwards-compatibility code
CentralAuth's policy is that master is compatible with core master only,
so let's drop some code that only exists to support old versions:
- Pre-SessionManager compatibility code, including the 'centralauth-autoaccount' user right.
- Pre-CentralIdLookup compatibility code.
- OAuth hooks that were needed before CentralIdLookup.
- GlobalUserPage hook that was needed before CentralIdLookup.
- Functions deprecated when the API help i18n was added.
- Stuff deprecated when the API token handling was redone.