Purges a field instance record from the database.
This function assumes all data for the instance has already been purged and should only be called by field_purge_batch().
$instance: The instance record to purge.
function field_purge_instance($instance) {
db_delete('field_config_instance')
->condition('id', $instance['id'])
->execute();
// Notify the storage engine.
$field = field_info_field_by_id($instance['field_id']);
module_invoke($field['storage']['module'], 'field_storage_purge_instance', $instance);
// Clear the cache.
field_info_cache_clear();
// Invoke external hooks after the cache is cleared for API consistency.
module_invoke_all('field_purge_instance', $instance);
}