Fix incorrect JsonStringEnumConverter on nullable strings in http-server-csharp

[aleksator]

TypeSpec definition:

nullableString: string | null;

produces

[JsonConverter(typeof(JsonStringEnumConverter))]
public string NullableString { get; set; }

when generated with http-server-csharp. The JsonStringEnumConverter is incorrectly applied because string | null is detected as a "string enum" union type. This always fails in runtime on deserialization.

This PR removes JsonConverter, generating

public string NullableString { get; set; }

When built on top of #9439, the generated code becomes:

public string? NullableString { get; set; }