Form submission handler for node_form_block_custom_block_delete_alter().
node_form_block_custom_block_delete_alter()
function node_form_block_custom_block_delete_submit($form, &$form_state) {
db_delete('block_node_type')
->condition('module', 'block')
->condition('delta', $form_state['values']['bid'])
->execute();
}