[auth] Remove unused `delete_account()` function

Summary:
This was added by Karim in 7d9244507bfc0a62b300ae036879c94d8c7682d2,
but the last reference to it was subsequently removed by Charles in
e5d98fdf2cf8bca566a822bb54f4a6ad18ce63b5.

Test Plan: Nothing much.  Nothing calls this, as far as grep tells me.

Reviewers: charles, emfree, khamidou

Reviewed By: emfree

Projects: #cypress

Differential Revision: https://review.inboxapp.com/D568
[auth] Rename private functions: auth_account, supports_condstore

Summary:
Make it clear that these functions are not part of the auth-module
interface.

Test Plan: Unit tests and running the app (as part of a larger test)

Reviewers: emfree, charles

Reviewed By: charles

Subscribers: kav-ya

Projects: #cypress

Differential Revision: https://review.inboxapp.com/D569
[auth] provider_info now accepts an email address argument

Test Plan: Added a unit test.  Ran the unit tests.

Reviewers: kav-ya, charles, emfree

Reviewed By: emfree

Projects: #cypress

Differential Revision: https://review.inboxapp.com/D570