bpo-39543: Remove unused _Py_Dealloc() macro (GH-18361)

The macro is defined after Py_DECREF() and so is no longer used by
Py_DECREF().

Moving _Py_Dealloc() macro back from cpython/object.h to object.h
would require to move a lot of definitions as well: PyTypeObject and
many related types used by PyTypeObject.

Keep _Py_Dealloc() as an opaque function call to avoid leaking
implementation details in the limited C API (object.h): remove
_Py_Dealloc() macro from cpython/object.h.