Required literals should not be in method signatures

[JoshLove-msft]

Required literals have only one allowed value and they must be specified in the request. Therefore they should be excluded from service methods. Similar to #4747

[markcowl]

@JoshLove-msft Is this intended as an issue for a class of emitters (like client emitters)? Or is this a request for what is represented in the type graph?

[JoshLove-msft]

Sorry - forgot to tag this as csharp client emitter.

[JoshLove-msft]

Though I think should be applicable across languages - I will see what other languages are doing.

[JoshLove-msft]

We ended up leaving these in for evolvability.