Function remove_member 

pub fn remove_member(
    murk: &mut Murk,
    name: &str,
    member_pubkey: &str,
    operator_pubkey: &str,
) -> Result<bool, MurkError>

Remove a member from a group. The operator must be a member. Refuses to remove the last member (the group’s secrets would become unreadable). Returns true if the member was removed (false if not present).