R1:40946248280e

Remove an ancient no-op check for duplicated external accounts

Summary:
Ref T13493. This check was introduced in D4647, but the condition can never be reached in modern Phabricator because the table has a unique key on `<accountType, accountDomain, accountID>` -- so no row can ever exist with the same value for that tuple but a different ID.

(I'm not entirely sure if it was reachable in D4647 either.)

Test Plan: Used `SHOW CREATE TABLE` to look at keys on the table and reasoned that this block can never…