Remove OAuth token/expiry interfaces

Summary:
Ref T1536. Currently, we store OAuth tokens along with their expiry times and status. However, all we use this for is refreshing profile pictures and showing a silly (and probably somewhat confusing) interface about token status.

I want to move this storage over to `PhabricatorExternalAccount` to make the cutover easier. Drop it for now, including all the profile image stuff (I plan to rebuild that in a more sensible way anyway).

Test Plan: Viewed screen; linked/unlinked accounts.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1536

Differential Revision: https://secure.phabricator.com/D6170