Reset iCloud tabs

Right now, I am selling my MacBook Air, and have therefore logged out of my iCloud account on that device. I also just did a clean install on my iMac, but forgot to log off iCloud before I wiped it clean. And my problem is that the iCloud tabs on those devices are still visible and won't get away (the iMac tabs are both for the device before and after the clean install—also shown as two different devices).

Any suggestions to how to get rid of them, since they're not part of my iCloud devices anymore?