remove$\_$friend:$$This function takes a username, password, friend$\_$username, and social network as input and removes a $$friendship$$ between the two users. This means that the function mutates the original dictionary so each user is removed from each other$$s friend list in the input social network dictionary. This function should authenticate the user using the authenticate$\_$user method before modifying the input social network dictionary. If the authentication fails this method should return the error code ERROR$\_$FAILED$\_$AUTHENTICATION. This function should also check if friend$\_$username is in the username$$s friend list, and return the error code ERROR$\_$FRIEND$\_$NOT$\_$FOUND if not. If the operation is successful this method should return nothing.