Remove "getApplicationTransactionObject()" from ApplicationTransactionInterface

Summary:
Depends on D19919. Ref T11351. This method appeared in D8802 (note that "get...Object" was renamed to "get...Transaction" there, so this method was actually "new" even though a method of the same name had existed before).

The goal at the time was to let Harbormaster post build results to Diffs and have them end up on Revisions, but this eventually got a better implementation (see below) where the Harbormaster-specific code can just specify a "publishable object" where build results should go.

The new get...Object semantics ultimately broke some stuff, and the actual implementation in Differential was removed in D10911, so this method hasn't really served a purpose since December 2014. I think that broke the Harbormaster thing by accident and we just lived with it for a bit, then Harbormaster got some more work and D17139 introduced "publishable" objects which was a better approach. This was later refined by D19281.

So: the original problem (sending build results to the right place) has a good solution now, this method hasn't done anything for 4 years, and it was probably a bad idea in the first place since it's pretty weird/surprising/fragile.

Note that Comment objects still have an unrelated method with the same name. In that case, the method ties the Comment storage object to the related Transaction storage object.

Test Plan: Grepped for getApplicationTransactionObject, verified that all remaining callsites are related to Comment objects.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T11351

Differential Revision: https://secure.phabricator.com/D19920