R1:84b5ad09e6e2

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…