There is some modification in your code, insted of this class .taxonomy-category tr:hover .row-actions
apply css on this class .taxonomy-category .row-actions span.delete
, it willl work.
Here is whole code.
add_action('admin_head', 'hide_category_buttons');
function hide_category_buttons() {
echo '<style>
.taxonomy-category .row-actions span.delete {
visibility: hidden;
}
</style>';
}