Simple all you need to do is edit this part of the code :
function editable_roles( $roles ){
if( isset( $roles['administrator'] ) && !current_user_can('administrator') ){
unset( $roles['administrator']);
}
return $roles;
}
and change it to
function editable_roles( $roles ){
//don't change anything if current user is admin
if (current_user_can('administrator')) {
return;
}else{
if( isset( $roles['administrator'] ) && !current_user_can('administrator') ){
unset( $roles['administrator']);
//here you add "unset( $roles['role_to_remove']);" for each role you wish to remove
}
return $roles;
}