D8 - Delete all instances of a field in an entity type

delete_field_instances.php

use Drupal\field\Entity\FieldStorageConfig;
use Drupal\field\Entity\FieldConfig;

$entity_type = 'node';
$bundles = entity_get_bundles('node');
$fields = ['field_phone', 'field_fax'];

foreach ($bundles as $bundle) {
  foreach ($fields as $field_name) {
    $field = FieldConfig::loadByName($entity_type, $bundle, $field_name);
    if (!empty($field)) {
      $field->delete();
    }
  }
}

foreach ($fields as $field_name) {
  $field_storage = FieldStorageConfig::loadByName($entity_type, $field_name);
  if (!empty($field_storage)) {
    $field_storage->delete();
  }
}