[C#] minor improvement to reduce unused using statements

[yowl]

No description provided.

[jsturtevant]

nice! This has been on the list under #713 "fix imports so they are only included if needed"

[yowl]

There's more 2 do here, but its a start.

[jsturtevant]

Seem the tests are passing. Is there more to do?

[jsturtevant]

Why the todo here?

[yowl]

Because its not perfect, the Intefaces in particular still have unused usings, e.g. BindingsWorld.wit.imports.foo.bar.IMyImport.cs has

// Generated by `wit-bindgen` 0.34.0. DO NOT EDIT!
// <auto-generated />
#nullable enable

using System.Text;
using System.Runtime.InteropServices;

namespace BindingsWorld.wit.imports.foo.bar;

public interface IMyImport {

    public class Address {
        public readonly uint field;
        public readonly bool attribute;

        public Address(uint field, bool attribute) {
            this.field = field;
            this.attribute = attribute;
        }
    }

}

Where neither using is required. The generation of the using statements is shared between files which makes it slightly tricky to get this last bit perfect.