Remove KeyCDN nag when API key applied

We love CDN Enabler and KeyCDN. I was surprised, however, that the notification nag on the CDN Enabler Settings page didn’t disappear after entering a valid API key and zone ID. (i.e. “Combine CDN Enabler with KeyCDN for even faster WordPress performance.”)

This is especially relevant on client websites when the client sees the nag and calls support: “You told me that our website has a CDN. So why is the CDN Enabler thingy telling us to get a CDN?”

Certainly not a huge issue, but a simple and nice way to make a fantastic plugin and service even better.