Fix merge 8e09e801 that resulted in duplicated code

A fragment in LUInstanceCreate.CheckPrereq() removed in commit ba147ff8 was
reintroduced in merge 8e09e801 due to a change in df28c49b.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>