Remove a stale source file entry from skia/BUILD.gn.

The SkCreateCGImageRef.cpp listing in skia/BUILD.gn is no longer
necessary, as Skia already includes the file in its internal build
rules. Chromium removed the file from its skia/BUILD.gn in
crrev.com/795295.

Change-Id: Ia009dffa5918a677f257f1dbbba6e31fe08e0213
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/103650
Reviewed-by: K. Moon <kmoon@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Auto-Submit: Lei Zhang <thestig@chromium.org>
Commit-Queue: K. Moon <kmoon@chromium.org>
diff --git a/skia/BUILD.gn b/skia/BUILD.gn
index 0eecb12..4835fb3 100644
--- a/skia/BUILD.gn
+++ b/skia/BUILD.gn
@@ -349,7 +349,6 @@
 
   if (is_ios) {
     frameworks += [ "ImageIO.framework" ]
-    sources += [ "//third_party/skia/src/utils/mac/SkCreateCGImageRef.cpp" ]
   }
 
   if (skia_support_pdf) {