Bug 26536: Fix incorrectly scoped variable

Bug 26536: Fix incorrectly scoped variable

During work on bug 26076 a variable scope mistake was made when
reverting the logic to use distinct queries for selected accountlines.

This patch restores the script scope of the varaible by removing the
superflous 'my' declaration.

Test plan
1/ Add a set of manual invoices
2/ Select a subset of those manual invoices to 'Writeoff selected'
3/ Confirm that only those selected invoices have been written off.

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>