Remove very old "vsDiff" data from commit update / diff extraction pipeline

Summary:
Ref T13276. This was introduced in D2586 to power a "trigger audits when the committed change does not match the reviewed change" feature. It was removed without ceremony in D15939. Broadly, rebases mean that this sort of feature can't really work like this and this approach is inherently unreliable; see also T182.

This property no longer has readers, and is unlikely to get any in the future since my planned pathway for "committed code must match reviewed code, modulo an automated rebase" is automating the rebase via "Land Revision", not comparing the diff text.

Remove this to simplify the flow of data here so that things in T13276 can be fixed more easily.

Test Plan: Grepped for vsDiff, no hits.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13276

Differential Revision: https://secure.phabricator.com/D20395