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.

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Review-Url: https://codereview.chromium.org/2273343002