Remove unneeded cpp file from CMake build. Rubber-stamp by Alexis Menard.

* CMakeLists.txt: Remove FFTFrameStub.cpp, since adding this file or not
to your build is a platform decision.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147432 268f45cc-cd09-0410-ab3c-d52691b4dbfc
Removing unneeded PLATFORM(NIX) check from FFTFrameStub.cpp.

Reviewed by: Trust me