radv: Only print "testing use only" message on GFX12+.

This message has been confusing users, especially now that popular toolkits such as Gtk started using a Vulkan renderer.

Printing a message on non-conformant implementations is also actually not required. So let's remote it.

We haven't fully finished the GFX12 implementation yet, but on all other hardware, RADV should work just fine, and is definitely not meant for "testing use only".