Bug 20144: [sql_modes] Fix GROUP BY clause in GetLetters

Bug 20144: [sql_modes] Fix GROUP BY clause in GetLetters

This subroutine is wrong and must be rewritten using
Koha::Notice::Templates.
Mainly because the DB structure is bad.
Meanwhile we remove the branchcode from the SELECT to get correct
results, it was not used by callers anyway.

Fix for:
'koha_kohadev.letter.module' isn't in GROUP BY

t/db_dependent/Letters.t

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>