dialog_widget: Remove redundant preventDefault calls.

Removes redundant `preventDefault` and `stopPropagation` calls for the
users of `dialog_widget.ts` module since we are already calling those
functions in the submit button click handler now.

web/src/stream_edit.js

@@ -452,8 +452,6 @@ export function initialize() {
     });
 
     function save_stream_info(e) {
-        e.preventDefault();
-        e.stopPropagation();
         const sub = get_sub_for_target(e.currentTarget);
 
         const url = `/json/streams/${sub.stream_id}`;