DEV: Refactor rp_id and rp_name (#23339)

They're both constant per-instance values, there is no need to store them
in the session. This also makes the code a bit more readable by moving
the `session_challenge_key` method up to the `DiscourseWebauthn` module.

spec/requests/session_controller_spec.rb

@@ -120,7 +120,7 @@ RSpec.describe SessionController do
           expect(response_body_parsed["challenge"]).to eq(
             DiscourseWebauthn.challenge(user, secure_session),
           )
-          expect(DiscourseWebauthn.rp_id(user, secure_session)).to eq(Discourse.current_hostname)
+          expect(DiscourseWebauthn.rp_id).to eq(Discourse.current_hostname)
         end
       end
     end