stream_settings: Fix error on stream setting update when compose closed.

Previously, when stream settings were updated, without opening the
compose box, assertion error were thrown.

This error was a result of a regression in compose_recipient, due to an
extra assert statement added during its typescript migration in
25ff0d4418.

This is fixed by removing the statement.

web/src/compose_recipient.ts

@@ -226,7 +226,6 @@ export function on_compose_select_recipient_update(): void {
     if (curr_message_type === "stream") {
         // Update stream name in the recipient box.
         const stream_id = compose_state.stream_id();
-        assert(stream_id !== undefined);
         update_recipient_label(stream_id);
     }