[rust png] Remove `lukasza` and `fmalita` from `.../libpng/OWNERS`.

Chrome no longer depends on `libpng` (see https://crrev.com/c/7108950
which refers in the CL description to earlier CLs which removed the
dependency from Chromium configuration of Skia and PDFium).  Therefore
this CL updates `//third_party/libpng/OWNERS` to reflect the remaining
dependencies.

Bug: 443128323
Change-Id: If02943408ab721db804908f2bd15d023b4d7897c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7198462
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Reviewed-by: Tom Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1549857}
NOKEYCHECK=True
GitOrigin-RevId: 17f6d1dc9b186f3659bdedaea4079e20fb9616e3