Add this code to Custom.css file
.cart-summary-voucher {
transition: none !important;
}
Edit History
in themes/your-theme/js/modules/blockcart/ajax-cart.js
In this file, you should look for the following line of code:
$('#blockcart-wrapper .blockcart-dropdown .cart-summary').hide().fadeIn(200);
in the code simply remove the .fadeIn(200) method call, like this:
$('#blockcart-wrapper .blockcart-dropdown .cart-summary').hide();
This will hide the discount field without any transition effect.