API delete action for pledge payment risks data loss

In API v3, the 'delete' action on PledgePayment object takes the pledge payment id and sends it to CRM_Pledge_BAO_Payment::deletePayments( $id ), which method expects a pledge id and wants to delete all payments from that pledge.

At this point there is no BAO method for deleting individual pledge payments.

The attached patch provides that method and points the API to it when deleting a pledge payment.