You might have signed up for the 15 day trial of the sync service instead of just clicking ‘Just use on this device’ when you first opened the app.
No worries, there are 2 ways to get rid of this dialog.
1) Just let the app expire, it will disappear after 15 days
2) Disable it by:
- Going into setting in the app by clicking on the settings icon in the lower left corner.
- Click ‘Sync Options’
- Click ‘Offline Only’