[CMake] Clean up FAST_MOBILE_SCROLLING
https://bugs.webkit.org/show_bug.cgi?id=133342

Reviewed by Gyuyoung Kim.

FAST_MOBILE_SCROLLING was removed in r168726.

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


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@169416 268f45cc-cd09-0410-ab3c-d52691b4dbfc