Unable to remove login item from Users & Groups startup list
For some reason, Content Manager Assistant added the login item to /Library/Preferences/com.apple.loginitems.plist instead of ~/Library/Preferences/com.apple.loginitems.plist.

I was able to remove the login item by deleting /Library/Preferences/com.apple.loginitems.plist and restarting (logging out and back in wasn't enough).