modules/priming: don't warn against unloading it

I can't see sufficient motivation here.  The cache will be slightly
less ready, but it's not often that you need to contact a root server.

Most importantly, kresd must work well anyway, even with empty cache.
Also, the compiled-in address set of root servers should be quite
accurate - the NS set has never changed, and the last address change
was five years ago with just one of 26 records changing.