Bug 28216: Fix vendor list group by in serials statistics wizard

Bug 28216: Fix vendor list group by in serials statistics wizard

Bug 24940 added sort vendor list by aqbookseller.name but removed accidentally the group by.
Now vendors are repeated in the list.

Test plan :
1) Create several subscriptions for same vendor
2) Go to Reports > Serials
3) Check list of vendors
=> Without patch you see the same vendor several times
=> With patch you seen it once
4) Check sort is still on vendor name

Signed-off-by: Samu Heiskanen <samu.heiskanen@hypernova.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>