This call seems to be unnecessary and removing it allows usage with Podman.

See https://github.com/docker/cli/blob/b0343d9104a1794c3252fa45b985710d6b6afc05/cli/command/container/exec.go#L111

This might be useful when a user wants to use a different network to run
the registry, especially for Podman users.

This patch avoids adding such a flag to clusterCreate to avoid polluting
the arguments for that command. A better long-term solution would be to
create a new network for the registry.

- network is not ambiguous for registry, so it can just be network instead of DefaultNetwork
- DefaultNetwork as a type however could be ambiguous in that it could be confused for k3d's default network (i.e. `k3d cluster create` without a name would create the "default" `k3d-k3s-default` network)