Revert "GN: define SK_XML to indicate :xml is enabled."

Revert "GN: define SK_XML to indicate :xml is enabled."

This reverts commit 7234448c11b0493a072723acd185dae986103df6.

Reason for revert: this enabled tests which are making the bots flaky.

Original change's description:
> GN: define SK_XML to indicate :xml is enabled.
>
> Without this, nothing ever defines SK_XML for dm/DM.cpp, dm/DMSrcSink.cpp, tests/SkDOMTest.cpp or tests/SkPEGTest.cpp.
>
> GYP does something similiar in gyp/xml.gyp.
>
> BUG=skia:
>
> GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4500
>
> Change-Id: I93322722d6d86f5802495d7a2edad2f82ba36f69
> Reviewed-on: https://skia-review.googlesource.com/4500
> Reviewed-by: Hal Canary <halcanary@google.com>
> Commit-Queue: Eric Boren <borenet@google.com>
> Commit-Queue: Mike Klein <mtklein@chromium.org>
>

TBR=mtklein@chromium.org,borenet@google.com,halcanary@google.com
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Change-Id: I46c198f4985f6e7f96c07b3100a3877ba46fdb18
Reviewed-on: https://skia-review.googlesource.com/4526
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Klein <mtklein@chromium.org>