Remove all readers and all nontrivial writers for "accountType" and "accountDomain" on "ExternalAccount"

Summary:
Depends on D21018. Ref T13493. Ref T6703. The "ExternalAccount" table has a unique key on <accountType, accountDomain, accountID> but this no longer matches our model of reality and changes in this sequence end writes to accountID.

Remove this key.

Then, remove all readers of accountType and accountDomain (and all nontrivial writers) because none of these callsites are well-aligned with plans in T6703.

This change has no user-facing impact today: all the rules about linking/unlinking/etc remain unchanged, because other rules currently prevent creation of more than one provider with a given "accountType".

Test Plan:

• Linked an OAuth1 account (JIRA).
• Linked an OAuth2 account (Asana).
• Used bin/auth refresh to cycle OAuth tokens.
• Grepped for affected symbols.
• Published an Asana update.
• Published a JIRA link.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13493, T6703

Differential Revision: https://secure.phabricator.com/D21019