[http-client-csharp] Adopt license changes from TCGC when it is ready

[ArcturusZhang]

We need to receive and handle the information of license from TCGC
PR developing here: Azure/typespec-azure#2353

[ArcturusZhang]

When we have this from TCGC, maybe this LicenseString property in CodeModelPlugin could be removed.

[JoshLove-msft]

Needed for GA

[JoshLove-msft]

Confirmed that license.txt is not required.