Sliced Invoices: hide "adjust (%)" field from admin quote/invoice pages

functions.php

add_action( 'sliced_after_line_items', 'sliced_admin_hide_the_adjust_field', 10, 2 );
function sliced_admin_hide_the_adjust_field( $line_items_group_id, $line_items ) {
	// for historical reasons the "adjust(%)" field is called "tax" internally,
	// although it is no longer used for this purpose.
	return $line_items->remove_field( 'tax', $line_items_group_id );
}