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/public.bzl b/public.bzl
index 39926b5..ddd8c6a 100644
--- a/public.bzl
+++ b/public.bzl
@@ -567,7 +567,7 @@
     "PNG_SKIP_SETJMP_CHECK",
     "SK_BUILD_FOR_UNIX",
     "SK_SAMPLES_FOR_X",
-    "SK_SFNTLY_SUBSETTER",
+    "SK_PDF_USE_SFNTLY",
     "SK_CODEC_DECODES_RAW",
     "SK_HAS_GIF_LIBRARY",
     "SK_HAS_JPEG_LIBRARY",