Bug 21621: Remove incorrect GROUP BY from tools/letter.pl

Bug 21621: Remove incorrect GROUP BY from tools/letter.pl

Fix:
  'koha_kohadev.letter.name' isn't in GROUP BY

DBD::mysql::db selectall_arrayref failed: 'koha_kohadev.letter.name' isn't in GROUP BY [for Statement "SELECT branchcode, module, code, name, branchname
            FROM letter
            LEFT OUTER JOIN branches USING (branchcode)
     GROUP BY branchcode,module,code ORDER BY module, code, branchcode"] at /home/vagrant/kohaclone/tools/letter.pl line 383.

Test plan:
- Copy some default notice templates for specific libraries
- List the different notice templates
- Confirm that notices appear only one, with "all libraries" you will see all

Signed-off-by: Pierre-Marc Thibault <pierre-marc.thibault@inLibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>