Code cleanup.

Two small code cleanup.

The first one remove COGLFragmentShaderCombiner class (inherited from COGLColorCombiner) from both OGLES2FragmentShaders and OGLFragmentShaders because it is not used at all in favor of COGL_FragmentProgramCombiner.

The second code cleanup is just a weird code that create a variable that is not used at all actually.

EDIT: I should have write all of this in the commit message instead of a small "remove unused code", sorry for that.