This reverts commit d90e221a17. Reverts #6489 since it introduced problems for people having [*]Auto sync now and in the furture & Action when wifi off: prompt.
The 'KOSync' plugin will synchronize furthest reading progress
across different koreader devices after users registering their
devices.
The synchronizing service is open-sourced as the project
[koreader/koreader-sync-server](https://github.com/koreader/koreader-sync-server).