Construct 2 was first released 9 years ago, in February 2011. Back then, Flash was still a dominant technology on the web; Microsoft's latest browser was Intern...
We have no power to remove things from people's Steam libraries. C2 will always be there until you remove it yourself.
The discount can only be used once, on your first payment (annual or monthly). Your discount will be biggest if you use it for an annual payment.