Some functions are not used in HeaderSchema.cs

As title. In class
```
[BestFriend]
internal abstract class FeatureNameCollection : IEnumerable
```
those functions
```
private FeatureNameCollection()
{
}

public static FeatureNameCollection Create(string[] names)
{
return Create(Utils.Size(names), names);
}

public static FeatureNameCollection Create(RoleMappedSchema schema)
{
…
}

public static void Save(ModelSaveContext ctx, in VBuffer> names)
{
…
}

public bool TryLookup(string name, out int index)
{
…
}
```
are not used.