Bug 26593: Remove _get_discount_from_rule

Bug 26593: Remove _get_discount_from_rule

This patch remove the private sub used in GetIssuingCharges in favor
of get_effective_rule

It corrects the wrong precedence for rules and adds tests to cover this
subroutine

NOTE: the 'branch' for the discount will be determined by the signed in branch,
    this is a bug to be fixed in the future

To test:
1 - Define a rentalcharge for an itemtype
2 - Define a 10% discount for library A, category A, all itemtypes
3 - Define a 50% discount for all libraries, category A, same itemtype
4 - Attempt to checkout an item from library A of the matching itemtype
5 - The 50% discount is applied
6 - Apply patches
7 - Attempt to checkout an item from library A of the matching itemtype
8 - The 10% discount is applied

prove -v t/db_dependent/Circulation.t

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>