Remove "passwordHash" and "passwordSalt" from User objects

Summary:
Ref T13043. After D18903, this data has migrated to shared infrastructure and has no remaining readers or writers.

Just delete it now, since the cost of a mistake here is very small (users need to "Forgot Password?" and pick a new password).

Test Plan: Grepped for passwordHash, passwordSalt, and variations.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13043

Differential Revision: https://secure.phabricator.com/D18904