Delete a device from iCloud
HT5372: Using and troubleshooting iCloud Tabs

Removing Tabs from the Tab list
To remove a tab from the list of iCloud Tabs, you must close that tab in Safari on the supported device that you used to browse that website. Quitting Safari, putting your Mac to sleep, or shutting it down will not remove the tab from the list. If you no longer have access to the device that that you used to browse that tab, the tab will automatically be cleared from the list after 7 days, if not updated.

Just wait.