It looks like the answer offered hides the Add to Cart, and the Price as well. If you only want to hide the Quantity input field but keep all the rest, you can add this to the custom CSS editor:
.product-quantity-input {
display:none !important;
}
This will affect products site wide, so if you need this for only a specific product page, you can add this in style tags via the page header code injection:
<style>
.product-quantity-input {
display:none !important;
}
</style>
