The “Get Windows 10” icon in the tray has been installed as part of the KB3035583 Windows update.
To remove that icon permanently uninstall this update in the windows software center. The Windows update function might offer this update again in future, so don't forget to hide it in Windows update so that it does not come back.
If you still see the icon after uninstalling and a reboot, then check if there are still files of the update in C:\Windows\System32\GWX. If that's the case, delete this directory.