Implements hook_field_storage_delete_instance().
This function simply marks for deletion all data associated with the field.
function field_sql_storage_field_storage_delete_instance($instance) {
$field = field_info_field($instance['field_name']);
$table_name = _field_sql_storage_tablename($field);
$revision_name = _field_sql_storage_revision_tablename($field);
db_update($table_name)
->fields(array(
'deleted' => 1,
))
->condition('entity_type', $instance['entity_type'])
->condition('bundle', $instance['bundle'])
->execute();
db_update($revision_name)
->fields(array(
'deleted' => 1,
))
->condition('entity_type', $instance['entity_type'])
->condition('bundle', $instance['bundle'])
->execute();
}