Remove the USE_GTK2=1 branch in OptionsGTK.cmake. This used to determine
GTK+ and GDK libraries and include directories for the GTK+ 2 dependency,
but we now only support GTK+ 3. The GTK+ 2 dependency is still required
by the plugin process, but it is searched and utilized separately.

Rubber-stamped by Carlos Garcia Campos.

* Source/cmake/OptionsGTK.cmake:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@170052 268f45cc-cd09-0410-ab3c-d52691b4dbfc