smtp_forward: remove redundant outbound hook

[gramakri]

Expected behavior

When using smtp_forward for outbound emails, the mail gets delivered via the SMTP mail delivery queue (just like normal emails). I expect delivered hook to be called after mail is delivered, but it's not.

Observed behavior

Mail is delivered via the queue_outbound hook. Because, it ties into this hook, if a mail gets delivered, the delivered hook is never called.

Instead, by relying on the get_mx hook, the email flow is now the same for email when using smtp_forward or not.