Remove the promotion code filed if none of the events within the cart have a promotion available.

example.php

<?php //Please do not include the opening PHP tag if you already have one 

function tw_remove_promo_input( $before_payment_options) {
    //If the promotions add-on is not active we don't need any of this to run.
    if( class_exists('EEM_Promotion') ) {
        $checkout = EE_Registry::instance()->SSN->checkout();
        if ( $checkout instanceof EE_Checkout ) {
            $transaction = $checkout->transaction;
            if ( $transaction instanceof EE_Transaction ) {
                //Create an array to hold the event ID's.
                $EVT_IDs = array();

                //Pull the event ID's from the registrations.
                foreach ( $transaction->registrations() as $registration ) {
                    if ( $registration instanceof EE_Registration ) {
                        $event = $registration->event();
                        if ( $event instanceof EE_Event ) {
                            $EVT_IDs[] = $event->ID();
                        }
                    }
                }
                //We only need each event ID once.
                $EVT_IDs = array_unique($EVT_IDs);
                
                //Pull any promotions linked to the events in the cart.
                $promotions = EEM_Promotion::instance()->count(
                    array(
                        array(
                            'Promotion_Object.POB_type' => 'Event',
                            'Promotion_Object.OBJ_ID' => array( 'IN', $EVT_IDs )
                        )
                    )
                );
                
                //No promtions? Remove the function to add the prmo submit.
                if( $promotions == 0 ) {
                    remove_filter('FHEE__EE_SPCO_Reg_Step_Payment_Options___display_payment_options__before_payment_options', array('EED_Promotions', 'add_promotions_form_inputs') );
                }
            }
        }
    }
    return $before_payment_options;
}
add_filter('FHEE__EE_SPCO_Reg_Step_Payment_Options___display_payment_options__before_payment_options', 'tw_remove_promo_input', 9 );