The function to remove a user from one or more groups is called remove_user_from_discourse_group, you have called it remove_user_to_discourse_group.
The functions for adding and removing a user from one or more groups are here: wp-discourse/lib/utilities.php at 99325e15190f3a705284dbf582f1c4b2c0b21492 · discourse/wp-discourse · GitHub.