Revert "[rust png] Reinforce `testonly`-ness of some users of `libpng`."

This reverts commit e800913378944238d93efb337e76afb44af08dfd.

Reason for revert: Causing some bots failure: https://ci.chromium.org/ui/p/chromium/builders/ci/Libfuzzer%20Upload%20iOS%20Catalyst%20Debug/116009/overview
Or https://ci.chromium.org/ui/p/chromium/builders/ci/ios-catalyst/201046/overview

Original change's description:
> [rust png] Reinforce `testonly`-ness of some users of `libpng`.
>
> This CL extracts a separate `third_party/libpng:libpng_for_testonly`
> target which:
>
> 1) Is a `group` that proxies/forwards to the `libpng` target by listing
>    it as its `public_deps`
> 2) Has limited visibility (based on the new `libpng_testonly_visibility`
>    introduced in the refactored `third_party/libpng/visibility.gni`).
>    The new visibility is almost unchanged, except for explicitly
>    spelling out some targets instead of using wildcards
> 3) Is marked as `testonly`
>
> Bug: 443128323
> Change-Id: I15626f7ece79304f9649057e7e215c3e72d15b92
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6976870
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1521521}

Bug: 443128323
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: I106b084528894950a44f52ce7e84c002c54a007e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6990712
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Owners-Override: Yi Gu <yigu@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Yi Gu <yigu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1521554}
NOKEYCHECK=True
GitOrigin-RevId: 7fa4c2d7ab42a48247ce8e0290cbb7e854990f1e