Don't generate `Typespec.` using

[timotheeguerin]

using Typespec.Http;
using Typespec.Rest;

should be replaced with

using Http;
using Rest;

[pshao25]

I saw the http library itself is still using Typespec.Http https://github.com/microsoft/typespec/blob/main/packages/http/lib/auth.tsp. Why we want to delete TypeSpec.? And how about OpenAPI and other libraries?

[timotheeguerin]

Typespec. In using it is optional(There is an implicit using Typespec; in every file. The current pattern is to omit it.