Remove state field from woocommerce checkout form.

plugin.php

<?php
/**
 * Remove state field from woocommerce checkout form.
 *
 * @see https://github.com/woocommerce/woocommerce/blob/3.1.2/includes/class-wc-countries.php#L629
 *
 * @param array $fields Array of default address fields.
 */
function jeherve_remove_state_field( $fields ) {
	unset( $fields['state'] );

	return $fields;
}
add_filter( 'woocommerce_default_address_fields', 'jeherve_remove_state_field' );

And how to customize that if, for example I'd like to hide only for specific countries (eg. France and Germany) ?

That's not something you'll be able to achieve via this filter, since you'll need to change the value as the customer enters their contact info. I would recommend that you look for plugins and solutions that allow you to define dynamic checkout fields, such as https://woocommerce.com/products/conditional-checkout-fields-for-woocommerce/

Thank You so much jeherve