[rust] Remove obsolete comment from the update script

The comment was a leftover from crrev.com/1276926

Bug: none
Change-Id: Ia9e301b925ce33b905ea458d07c262a79ce12511
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5425021


Auto-Submit: Hans Wennborg <hans@chromium.org>
Commit-Queue: Zequan Wu <zequanwu@google.com>
Reviewed-by: Zequan Wu <zequanwu@google.com>
Cr-Commit-Position: refs/heads/main@{#1283145}