cogl-path uses types from glu.h, but to avoid a build dependency on glu,
it kept a minified copy of glu.h in tree. Drop this file and just use
the actual glu.h. To avoid linking to libGLU.so, just use the
includepath, instead of actually adding glu as a real dependency.

This means we can remove an includepath meant to make it possible to
include <GL/glu.h>.