Bug 7505 Superlibrarian can't export all biblios when IndependantBranches is enabled

Bug 7505 Superlibrarian can't export all biblios when IndependantBranches is enabled

When a Koha user has superlibrarian permissions, and if
IndependantBranches is enabled, in Tools > Export bibliographic and
holdings, this user can't export all biblio records. He will just export
the biblio from the branch he belongs to.

This patch fix that.

To test:

  - Do a ALL records export as superlibrian on a multi-branches Koha instance
  - Examine the resulting MARC file with the tool of your choice (MArcEdit,
    yaz-marcdump, etc.)
  - You will see that 952/995 contains exlusively items from your branch (as
    Koha user)

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>