[rust png] Remove unused/obsolete `SK_ENCODE_PNG` define.

AFAICT the `SK_ENCODE_PNG` is

1. Defined in Pdfium's `skia/BUILD.gn` [1]
2. Mentioned in comments in `third_party/pdfium/skia/BUILD.gn` and
   `third_party/skia/RELEASE_NOTES.md`.
3. Not used anywhere else

Therefore let's remove the definition from Pdfium, because it seems
unused and unnecessary.  FWIW the definition has been introduced in
https://pdfium-review.googlesource.com/c/pdfium/+/101450

[1] https://source.chromium.org/chromium/chromium/src/+/main:third_party/pdfium/skia/BUILD.gn;l=58;drc=7be0974785d088e89a910729893ffc88f581ff50

Bug: chromium:444045690
Change-Id: I54c068324e23cab0d52d3078113798703748bfec
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/135613
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
diff --git a/skia/BUILD.gn b/skia/BUILD.gn
index 62d63ca..e8543bf 100644
--- a/skia/BUILD.gn
+++ b/skia/BUILD.gn
@@ -55,7 +55,6 @@
   include_dirs = [ "//third_party/skia" ]
 
   defines = [
-    "SK_ENCODE_PNG",
     "SK_USER_CONFIG_HEADER=\"../../skia/config/SkPdfiumUserConfig.h\"",
     "SK_DISABLE_LEGACY_INIT_DECODERS",
     "SK_DISABLE_LEGACY_PNG_WRITEBUFFER",