Remove MeshPy::coarsen() and code using GTS

FC_USE_GTS not used since before:
1162aaea88 ("Remove references to gts and ode", 2012-07-05)

GTSAlgos.h and GTSAlgos.cpp removed in:
db5c8f7db8 ("[Mesh] remove GTSAlgos", 2022-11-11)

MeshPy::coarsen() added not later than 2011 and never implemented.

src/Mod/Mesh/App/MeshPyImp.cpp

@@ -593,15 +593,6 @@ PyObject* MeshPy::section(PyObject* args, PyObject* kwds)
     return Py::new_reference_to(outer);
 }
 
-PyObject* MeshPy::coarsen(PyObject* args)
-{
-    if (!PyArg_ParseTuple(args, "")) {
-        return nullptr;
-    }
-    PyErr_SetString(PyExc_NotImplementedError, "Not yet implemented");
-    return nullptr;
-}
-
 PyObject* MeshPy::translate(PyObject* args)
 {
     float x {};