use Drupal\field\Entity\FieldStorageConfig;
use Drupal\field\Entity\FieldConfig;
/**
* Implements hook_uninstall().
*/
function mymodule_uninstall($is_syncing) {
$field_storage = FieldStorageConfig::loadByName('user', 'field_mymodule_notify');
if ($field_storage) {
$field_storage->delete();
}
$field = FieldConfig::loadByName('user', 'user', 'field_mymodule_notify');
if ($field) {
$field->delete();
}
}