frontend: Remove html_submit_button paramter passed to dialog_widget.

We used html_submit_button to pass text to be present in the modal
submit button. There are only two possible options as of now -
"Confirm" and "Save changes" and the correct one can be determined
using is_confirm_modal parameter. So, we remove this paramter for
now and we can add it later if we have more type of modals using
this widget.

static/js/stream_create.js

@@ -441,7 +441,6 @@ export function set_up_handlers() {
                 parent: modal_parent,
                 html_heading: $t_html({defaultMessage: "Large number of subscribers"}),
                 html_body,
-                html_submit_button: $t_html({defaultMessage: "Confirm"}),
                 on_click: () => {
                     create_stream();
                 },