Bug 20763: Remove unecessary Koha::Patron fetch

Bug 20763: Remove unecessary Koha::Patron fetch

If $borrowernumber is not set, there is no userenv.
So let's pick the library code set in the userenv instead of fetching
the Koha::Patron->branchcode from DB

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit 7e025fde1ae976cb12d4593b33579e12f5b284ec)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>