stream_data: Remove stream_name param from add_sub().

We just get the stream_name from the sub struct now.

This mostly affects node tests.

The only place in real code where we called add_sub()
was when we initialized data from the server.

frontend_tests/node_tests/unread.js

@@ -31,7 +31,7 @@ const social = {
     subscribed: true,
     is_muted: false,
 };
-stream_data.add_sub('social', social);
+stream_data.add_sub(social);
 
 const zero_counts = {
     private_message_count: 0,