Stop "join project" from trying to write an inverse edge on Users

Summary: Now that Users implement PhabricatorApplicationTransactionInterface, we try to write an inverse edge. At least for now, we should retain the old behavior instead.

Test Plan:

• Unit tests which cover this stuff pass again.
• Grepped for other instanceof PhabricatorApplicationTransactionInterface, the all seemed either benign or irrelevant.

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D13215