Bug 16357: (QA follow-up) Only initialize Log4perl module

Bug 16357: (QA follow-up) Only initialize Log4perl module

Koha::Logger->get was setting up some extra variables
which are not used nor needed for the Plack::Middleware::Log4perl
middleware to work. According to documentation at
https://metacpan.org/pod/Plack::Middleware::Log4perl#SYNOPSIS
only running Log::Log4perl::init, enabling Log4Perl middleware and
setting up the logging category is enough. Koha::Logger->_init runs the
Log::Log4perl::init and enabling and setting category is handled
directly in plack.psgi.

Signed-off-by: Arthur Suzuki <arthur.suzuki@biblibre.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>