Acts when a field record is being purged.
In field_purge_field(), after the field configuration has been removed from the database, the field storage module has had a chance to run its hook_field_storage_purge_field(), and the field info cache has been cleared, this hook is invoked on all modules to allow them to respond to the field being purged.
$field: The field being purged.
function hook_field_purge_field($field) {
db_delete('my_module_field_info')
->condition('id', $field['id'])
->execute();
}