Remove the "PhabricatorRepositoryVCSPassword" class and table

Summary:
Ref T13043. After D18898, this has been migrated to new, more modern storage and no longer has any readers or writers.

One migration from long ago (early 2014) is affected. Since this is ancient and the cost of dropping this is small (see inline), I just dropped it.

I'll note this in the changelog.

Test Plan: Ran migrations, got a clean bill of health from storage status. Grepped for removed symbol.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13043

Differential Revision: https://secure.phabricator.com/D18899