[rust png] Trim `//third_party/libpng/visibility.gni`.

Recent CLs (https://crrev.com/c/7108442, https://crrev.com/c/7108605 and
https://crrev.com/c/7128187) stopped using those dependency edges.

Bug: 443128323
Change-Id: I13450d8ca855d85026c6ab3a1cfe7b3861ef023b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7108950
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1548758}
NOKEYCHECK=True
GitOrigin-RevId: decc5bea5d2747243c528c784229e46e1c4d3cb1