if scans find nothing, it is possible that the actual data is all gone. You should be able to see where the uninstall entry points to through registry:
Following directions will assume you know how to use regedit.
\\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall
It would be one of the entries under there, either by it's clsid or name, scroll down and check each one until you find the thing in question.
It should have following clues:
Display name (should match Cities Skylines)
Install date (should match the date on uninstall options)
Version (should be 1.0 as shown in uninstall options)
And.. what you would be interested in, where it supposedly is, path wise, allowing you to either find and delete rest or.. if it's already gone, simply delete the uninstall entry for it in the registry. (NOT whole uninstall path) causing it to vanish from the uninstall list.