[CMAKE] Remove SHADOW_DOM from cmakeconfig.h.cmake
https://bugs.webkit.org/show_bug.cgi?id=115712

Reviewed by Andreas Kling.

Nobody uses SHADOW_DOM in cmake.

* Source/cmake/WebKitFeatures.cmake:
* Source/cmakeconfig.h.cmake:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@149702 268f45cc-cd09-0410-ab3c-d52691b4dbfc