avoid template variable leakage when processing more than one recurring contribution in a session

This bug happens if you have more than one iATS recurring contribution and an earlier contribution is assigned to a soft credit, but one or more later contributions are not assigned to a soft credit.

The later contributions have the soft credit information inserted into their receipt because the email message template variables are not properly cleared.

iATS processes multiple recurring contributions in a single session via a cron job, so may be uniquely triggering this error (although the bug is with CiviCRM Core).