[CMAKE] Remove Nix from CMake scripts

https://bugs.webkit.org/show_bug.cgi?id=127264

Reviewed by Anders Carlsson.

.:

* CMakeLists.txt:
* Source/CMakeLists.txt:
* Source/cmake/FindEGL.cmake:
* Source/cmake/OptionsCommon.cmake:

Source/Platform:

* CMakeLists.txt: Removed.

Source/WebCore:

* CMakeLists.txt:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@162427 268f45cc-cd09-0410-ab3c-d52691b4dbfc