[Python] Don't exploit garbage collection hooks for memory regulator

Looking at the CPPInstance, I noticed that is claims to support cycling
garbage collection via the `Py_TPFLAGS_HAVE_GC`, but it doesn't stick
completely to the contract explained in the docs:

https://docs.python.org/3/c-api/gcsupport.html#c.PyObject_GC_Track

In particular, it breaks the constructor rules:

  * The memory for the object must be allocated using PyObject_GC_New or
    PyObject_GC_NewVar.

  * Once all the fields which may contain references to other containers
    are initialized, it must call PyObject_GC_Track().

The `tp_traverse` and `tp_clear` implementations actually do nothing
with the C Python API, so from the point of view of Python, there is no
reason to support cyclic garbage collection.

I think the CPPInstance tries to exploit the Python garbage collector to
trigger its own memory regulator code path early in `tp_clear`, but
breaking some of the Python C API contracts on the way. And the C++
object is also unregisterd from the memory regulator in `tp_dealloc`
anyway.

This might be the reason for the spurious failures in the CI, which
happen *between* two tests, hinting to garbage collector problems.
I think it's particularly problematic that the CPPInstance never
registers itself to the GC with `PyObject_GC_Track`, but then uses
`PyObject_GC_UnTrack`. That probably brings the garbage collector in a
bad state.

This reverts parts of the following CPyCppy commits from 2019:
  * wlav/CPyCppyy@e43759a
  * wlav/CPyCppyy@29eb42e

Author: guitargeek

bindings/pyroot/cppyy/CPyCppyy/src/CPPInstance.cxx

@@ -447,20 +447,8 @@ static CPPInstance* op_new(PyTypeObject* subtype, PyObject*, PyObject*)
 static void op_dealloc(CPPInstance* pyobj)
 {
 // Remove (Python-side) memory held by the object proxy.
-    PyObject_GC_UnTrack((PyObject*)pyobj);
     op_dealloc_nofree(pyobj);
-    PyObject_GC_Del((PyObject*)pyobj);
-}
-
-//----------------------------------------------------------------------------
-static int op_clear(CPPInstance* pyobj)
-{
-// Garbage collector clear of held python member objects; this is a good time
-// to safely remove this object from the memory regulator.
-    if (pyobj->fFlags & CPPInstance::kIsRegulated)
-        MemoryRegulator::UnregisterPyObject(pyobj, (PyObject*)Py_TYPE((PyObject*)pyobj));
-
-    return 0;
+    Py_TYPE(pyobj)->tp_free((PyObject*)pyobj);
 }
 
 //----------------------------------------------------------------------------
@@ -1086,11 +1074,10 @@ PyTypeObject CPPInstance_Type = {
     0,                             // tp_as_buffer
     Py_TPFLAGS_DEFAULT |
         Py_TPFLAGS_BASETYPE |
-        Py_TPFLAGS_CHECKTYPES |
-        Py_TPFLAGS_HAVE_GC,        // tp_flags
+        Py_TPFLAGS_CHECKTYPES,     // tp_flags
     (char*)"cppyy object proxy (internal)", // tp_doc
     (traverseproc)op_traverse,     // tp_traverse
-    (inquiry)op_clear,             // tp_clear
+    0,                             // tp_clear
     (richcmpfunc)op_richcompare,   // tp_richcompare
     0,                             // tp_weaklistoffset
     0,                             // tp_iter