M1: Unassign related products

unassign_related_product.php

<?php

include 'app/Mage.php';
Mage::app();

$collection = Mage::getModel('catalog/product')->getCollection();
foreach($collection as $_product){
    $product = Mage::getModel('catalog/product')->load($_product->getId());
    $linkData = array();

    Mage::getResourceModel('catalog/product_link')->saveProductLinks(
        $product, $linkData, Mage_Catalog_Model_Product_Link::LINK_TYPE_RELATED
    );
}