Remove SKS network from keyserver defaults
The keyservers from the SKS network were recently infected with poisoned certificates, as proclaimed here https://gist.github.com/rjhansen/67ab921ffb4084c865b3618d6955275f.
As a result of importing these certificates, some people can experience slow (to the point of being unusable) GPG tools. This is something that cannot be "fixed" in SKS keyservers, so let's at least drop them from the default keyservers we have in gcr (which are also used by seahorse).