extmod/nimble: Remove TODO comment about notify_custom freeing om.

The comments in NimBLE for ble_gattc_notify_custom() state that "This
function consumes the supplied mbuf regardless of the outcome.".  And
inspection of NimBLE code shows that this is the case.  So the comment can
be removed.

Signed-off-by: Damien George <damien@micropython.org>

extmod/nimble/modbluetooth_nimble.c

@@ -1030,7 +1030,6 @@ int mp_bluetooth_gatts_notify_send(uint16_t conn_handle, uint16_t value_handle,
     if (om == NULL) {
         return MP_ENOMEM;
     }
-    // TODO: check that notify_custom takes ownership of om, if not os_mbuf_free_chain(om).
     return ble_hs_err_to_errno(ble_gattc_notify_custom(conn_handle, value_handle, om));
 }