[http-client-csharp] remove the namespace configuration from the emitter

[ArcturusZhang]

Now we have to keep the namespace configuration because as a consumer of the emitter in this repo, autorest.csharp depends on it so tightly.
We need to figure out a way to remove this configuration and autorest.csharp to figure out what it should use as a value when there is not such a configuration defined in the input.

The configuration is defined here:

typespec/packages/http-client-csharp/emitter/src/emitter.ts

Line 99 in ccb5c4f

namespace: namespace,