I presume I have to register the airport scenery again in the FSX Add-On programme.
No, you will not. That's what the installer is for.
As I do not want any issues, if asked to remove Add-on Manager & Coult can I ignore this message?
You usually WANT to ignore the message, especially considering you have other products requiring the modules because, the message will also warn you that if you remove the modules, no products requiring them will work.
You reply YES to the removal question, when you need to do a clean reinstall for troubleshooting reasons.