Clean up `nextclade` directory

[joverlee521]

Copied from @j23414's comment

To clean up the WNV repo (not urgent) we could

1. Move a working historic dataset from https://github.com/nextstrain/WNV/tree/main/nextclade/dataset to https://github.com/nextstrain/WNV/tree/main/nextclade_data
2. Update this config path to "../nextclade_data"
3. Delete the nextclade folder entirely