Bug 27058: Remove confusing and unnecessary $biblionumber1 variable

Bug 27058: Remove confusing and unnecessary $biblionumber1 variable

The code is much more understandable now because with $biblio2 and
$biblionumber1 variables use to point to the same biblio one might
have thought $biblionumber1 points to $biblio1 which in this case is
not true. Let's just drop the extra variable because the object
notation of accessing is just as simple.

To test:
 1) prove t/db_dependent/Holds/DisallowHoldIfItemsAvailable.t => passes

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>