Use Purchase Log Action Links API for resending email receipts

See issue #1576

Deprecates the calling of `wpsc_purchlog_resend_email()` via a URL query in `admin_init` to resend email receipts and instead use the Purchase Log Action Link API.

`wpsc_purchlog_resend_email()` was only called internally for this purpose and checked `$_REQUEST['email_buyer_id']`

This functionality is now deprecated with `_wpsc_doing_it_wrong()` and `wpsc_purchlog_resend_email()` expects a purchase log ID to be passed as a parameter rather than checking for a $_REQUEST value.