Bug 34825: Remove Test::DBIx::Class from Letters.t

Bug 34825: Remove Test::DBIx::Class from Letters.t

Includes a tidy.

Test plan:
Run t/Letters.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 22aee4db67280da3a92713dae93e57737a94b303)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit 071c8ca79e34b16d12901f225c1fcfe6d8c5545d)
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>