Use real contribution tracking id for recurring GC

We were using a dummy contribution tracking ID to avoid some
mediawiki-dependent code paths, and ended up sending 123456 as the
merchant reference for a few days worth of recurring contributions.
This patch should look up the real tracking id and use that instead
of the dummy.

Change-Id: Idce6d10eb17497a212651a2b7daf6e8b61e6f144