Bug 16199: Remove C4::Ris::charconv

Bug 16199: Remove C4::Ris::charconv

This subroutine always returns what has been sent in parameter.
It is unecessary and can be removed.

Test plan:
  prove t/Ris.t
should not be noisy

Export a catalogue record in Ris should generate the same file with and
without this patch

NOTE: With/Without were identical in my testing.
      t/Ris.t is nicely silenced.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com
(cherry picked from commit 88dc70f7e691da61742f79c46c601de82a1d7e56)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>