Remove use of skia_core_defines

This was removed from Chromium in https://crrev.com/c/3831118

Change-Id: I394230f0bb278f12efc1de52c7361f096daeefb5
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/96570
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Auto-Submit: Kevin Lubick <kjlubick@google.com>
diff --git a/skia/BUILD.gn b/skia/BUILD.gn
index 0d7931a..ef014d5 100644
--- a/skia/BUILD.gn
+++ b/skia/BUILD.gn
@@ -418,7 +418,7 @@
 }
 
 skia_source_set("skia_core_and_effects") {
-  defines = skia_core_defines
+  defines = []
   sources = skia_core_sources
   sources += skia_effects_sources
   sources += skia_effects_imagefilter_sources