Bug 18613: Remove letter rule correctly as superlibrarian

Bug 18613: Remove letter rule correctly as superlibrarian

TEST PLAN

(reproducing bug)
1 - Set 'IndependantBranches' to enable
2 - Log in as Superlibrarian.
3 - Create 3 rules
    - 1 for all librairies (1)
    - 1 for the library of the superlibrarian (2)
    - 1 for another library (3)
4 - Try to delete them and see them and see that (3) isn't the good one

(checking patch)
5 - Apply patch
6 - Try again 3-
7 - Connect as not superlibrarian
8 - Try again 3-

Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>