Use the correct (synthetic) gateway_txn_id

Recurring GC donations all share an order id, so we append the -effort_id.

We have been omitting the "RECURRING" component of contribution.trxn_id forever.

Clean up small unused things.

Change-Id: I232b81758c63d236e79ed3893c6f69a2955adc93
Merge "Use the correct (synthetic) gateway_txn_id"