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 have any effect.

Maniphest Tasks: T13493

Differential Revision: https://secure.phabricator.com/D21012