[rclcpp] refactor Executor::add_callback_group to not need a node pointer

[wjwwood]

In #1218 the add_callback_group() method was added, but it required us to pass the node associated with the callback group as well, because it was still needed internally in the executor and because the callback group doesn't have a back reference to the node that created it.

We need to change this so it is no longer required because the library can have all this information and asking the user to provide it is redundant and can only lead to misuse, i.e. it has no practical value or serves no extra use case.