SkPDF: Stop `#include PREPROCESSOR_DEFINE` pattern

No more:
   #include SK_SFNTLY_SUBSETTER
   #include ZLIB_INCLUDE

Also, rename SK_SFNTLY_SUBSETTER to SK_PDF_USE_SFNTLY
to follow my pattern of prefixing SkPDF-specific defines
with 'SK_PDF_'.

The ZLIB_INCLUDE define is no longer is used by anyone.

TODO: rename Sfntly to something pronounceable.

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2273343002

Review-Url: https://codereview.chromium.org/2273343002
diff --git a/BUILD.gn b/BUILD.gn
index 6774890..bb451c3 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -285,10 +285,7 @@
 
   if (skia_use_sfntly) {
     deps += [ "//third_party/sfntly" ]
-    public_defines += [
-      # TODO(halcanary): make this the default; this is the value Android uses.
-      "SK_SFNTLY_SUBSETTER=\"sample/chromium/font_subsetter.h\"",
-    ]
+    public_defines += [ "SK_PDF_USE_SFNTLY" ]
   }
 }