[SCHEMA] Remove encrypted-password-storage schema from base Account object.

Summary:
Instead, Account subclasses for providers that use password auth (YahooAccount,
EASAccount) should just store a 'password' field directly.

Test Plan:
Tested the migration with one EAS account, and auth after applying
this patch with another. Both work.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D206