Revert of SkPDF: Optionally output PDF/A-2b archive format. (patchset #5 id:80001...

Revert of SkPDF: Optionally output PDF/A-2b  archive format. (patchset #5 id:80001 of https://codereview.chromium.org/1394263003/ )

Reason for revert:
SkMD5 is not really part of the Skia library. This is breaking the roll by using it, since Chromium doesn't build it.

Original issue's description:
> SkPDF: Optionally output PDF/A-2b  archive format.
>
> Note: this format does not yet pass validation tests.
>
> Add skia_pdf_generate_pdfa GYP flag.  Default to off for now.
> PDF/A files are not reproducable, so they make correctness
> testing harder.
>
> Turn the Metadata struct into te SkPDFMetadata struct.  This
> splits out a lot of functionality around both kinds of metadata.
>
> When PDF/A is used, add an ID entry to the trailer.
>
> Add SkPDFObjNumMap::addObjectRecursively.
>
> Test with
>
>     GYP_DEFINES=skia_pdf_generate_pdfa=1 bin/sync-and-gyp
>     ninja -C out/Release dm
>     out/Release/dm --config pdf --src skp gm -w /tmp/dm
>
> With skia_pdf_generate_pdfa=0, all PDFs generated from GMs and
> SKPs are identical.  With skia_pdf_generate_pdfa=1, all PDFs
> generated from GMs and SKPs render identically in Pdfium.
>
> BUG=skia:3110
>
> Committed: https://skia.googlesource.com/skia/+/939c0fe51f157104758bcb268643c8b6d317a530

TBR=tomhudson@google.com,halcanary@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:3110

Review URL: https://codereview.chromium.org/1398193002