Default attributes of WooCommerce variable products are stored as post meta in the database.
You can find them in the wp_postmeta table, where the post_id column is the post ID of the parent product (Variable product), and the meta_key column is _default_attributes.
You can clear and remove default attributes of all products by replacing all non-empty arrays.
To do so, open phpMyAdmin and select the database of your WordPress installation from the left panel and click on SQL tab. Then write the below SQL commands and press Go:
Don’t forget to backing up your database before executing any command
on phpMyAdmin
UPDATE `wp_postmeta` SET `meta_value`= 'a:0:{}' WHERE meta_key = '_default_attributes'