[rust png] Remove GN-level Blink => libpng dependencies.

Code-level `libpng` dependencies under `third_party/blink` are already
gone at this point (thanks to CLs like  https://crrev.com/c/6907475
and/or https://crrev.com/c/6931691).  Therefore this CL removes
remaining, obsolete GN-level dependency edges + tightens `visibility`.

Bug: 443128323
Change-Id: I9200cdfb1abd6e79e00655ed7c97352c709a16f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6970274
Auto-Submit: Łukasz Anforowicz <lukasza@chromium.org>
Owners-Override: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1519716}
NOKEYCHECK=True
GitOrigin-RevId: 8936b43bdf655c27337276f2230f00069da986d1