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 <14574f09dfa9b4e14759b88c3426a495a0e627b0@chromium.org>
> 

TBR=14574f09dfa9b4e14759b88c3426a495a0e627b0@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 <14574f09dfa9b4e14759b88c3426a495a0e627b0@chromium.org>
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@chromium.org>

rubenvb/skia,rubenvb/skia,google/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,rubenvb/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,rubenvb/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,google/skia,Hikari-no-Tenshi/android_external_skia,google/skia,HalCanary/skia-hc