How do I hook up the code to change Drupal module behaviour? - drupal

Question

Drupal does not allow me to delete extension (recurring_commerce) as some fields needs to be deleted manually. This issue is described in detalis below.
Solution is to add some code to a file as described in #11 of the issue below.
Basically I need to programmatically delete a field and I have never done it before. Writing kind of a hook (?).
I tried to paste the suggested code before, after the original code as well as replacing the original code with a suggested one.
Every time after changing the code I am restarting the server, running cron and clear cache.
Still I am unable to uninstall the exension from drupal as I have the following message:
The following reason prevents Commerce Recurring from being uninstalled:
The Billing period field type is used in the following fields: commerce_order.billing_period, commerce_order_item.billing_period
Details of the following issue:
please skip to #11
https://www.drupal.org/project/commerce_recurring/issues/2924965
Suggested code to apply:
function commerce_recurring_update_8101(&$sandbox) {
\Drupal::entityManager()->getStorage('field_config')->load('commerce_order.recurring.billing_period')->delete();
\Drupal::entityManager()->getStorage('field_config')->load('commerce_order.recurring.billing_schedule')->delete();
\Drupal::entityManager()->getStorage('field_config')->load('commerce_order.recurring.order_items')->delete();
\Drupal::entityManager()->getStorage('field_config')->load('commerce_order_item.recurring_standalone.billing_period')->delete();
\Drupal::entityManager()->getStorage('field_config')->load('commerce_order_item.recurring_standalone.subscription')->delete();
\Drupal::entityManager()->getStorage('field_config')->load('commerce_order_item.recurring_product_variation.billing_period')->delete();
\Drupal::entityManager()->getStorage('field_config')->load('commerce_order_item.recurring_product_variation.subscription')->delete();
}
function commerce_recurring_update_8102(&$sandbox) {
\Drupal::entityTypeManager()->getStorage('commerce_order_type')->load('recurring')->delete();
\Drupal::entityTypeManager()->getStorage('commerce_order_item_type')->load('recurring_standalone')->delete();
\Drupal::entityTypeManager()->getStorage('commerce_order_item_type')->load('recurring_product_variation')->delete();
}
Original code in my file:
/**
* Add the 'trial_starts' and "trial_ends" fields to subscriptions.
*/
function commerce_recurring_update_8101(&$sandbox) {
$fields = [];
$fields['trial_starts'] = BaseFieldDefinition::create('timestamp')
->setLabel(t('Trial starts'))
->setDescription(t('The time when the subscription trial starts.'))
->setRequired(FALSE)
->setDisplayOptions('view', [
'label' => 'hidden',
'type' => 'timestamp',
'weight' => 0,
])
->setDisplayOptions('form', [
'type' => 'datetime_timestamp',
'weight' => 0,
])
->setDisplayConfigurable('form', TRUE);
$fields['trial_ends'] = BaseFieldDefinition::create('timestamp')
->setLabel(t('Trial ends'))
->setDescription(t('The time when the subscription trial ends.'))
->setRequired(FALSE)
->setDisplayOptions('view', [
'label' => 'hidden',
'type' => 'timestamp',
'weight' => 0,
])
->setDisplayOptions('form', [
'type' => 'datetime_timestamp',
'weight' => 0,
])
->setDisplayOptions('form', [
'type' => 'datetime_timestamp',
'weight' => 0,
])
->setDisplayConfigurable('form', TRUE);
$update_manager = \Drupal::entityDefinitionUpdateManager();
foreach ($fields as $name => $storage_definition) {
$update_manager->installFieldStorageDefinition($name, 'commerce_subscription', 'commerce_recurring', $storage_definition);
}
}
/**
* Make the billing_schedule field required on subscriptions.
*/
function commerce_recurring_update_8102() {
$entity_definition_update = \Drupal::entityDefinitionUpdateManager();
$field_definition = $entity_definition_update->getFieldStorageDefinition('billing_schedule', 'commerce_subscription');
$field_definition->setRequired(TRUE);
$entity_definition_update->updateFieldStorageDefinition($field_definition);
}