Bug 28606: Remove $DEBUG and $ENV{DEBUG}

Bug 28606: Remove $DEBUG and $ENV{DEBUG}

We should remove the debug statements or use Koha::Logger when we want
to keep it.

Test plan:
Confirm that occurrences of remaining occurrences of DEBUG need to be
kept (historical scripts for instance)
Confirm that the occurrences removed by this patch can be removed
Confirm that the occurrences replaced by Koha::Logger are correct

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Looks good to me, noting a few minor points on BZ.

JD amended patch: replace "warn #Finished" with "#warn Finished", and
put the statement on a single line

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>