py/objslice: Remove long-obsolete comment about enhancing slice object.
Commit made this comment obsolete.
#if MICROPY_PY_BUILTINS_SLICE
-// TODO: This implements only variant of slice with 2 integer args only.
-// CPython supports 3rd arg (step), plus args can be arbitrary Python objects.
typedef struct _mp_obj_slice_t {
mp_obj_base_t base;
mp_obj_t start;
