Remove outdated todo list from the repository

- We have following data models in the libaccounts-qt
  - AccountsModel
  - ProvidersModel
  - AccountsServiceModel
- We don't need KDE signon-ui as there exists already Qt based signon-ui
- There exists already qt based singond oauth2 and password plugins
- ktp code is no-longer part of kaccounts-integration
- akonadi code is in work-in-progress
- kio-webdav already is running as kaccount plugin
- There already exists signon-kwallet-extension