Remove bitnodes.io from dnsseeds.

Sadly, bitnodes.io has shown lack of understanding of what
constitutes abusive behavior on the network and, as such, cannot
be supported in any way.

I know it doesnt really mean anything to remove them from this list but, if nothing else, it removes some small amount of credibility they might have.

See http://www.reddit.com/r/Bitcoin/comments/2pcsbr/new_bitnodes_feature_see_new_transactions/ for context (they're holding dedicated connections open to the whole network to show transaction propagation)