Bug 17931: Remove unused vars from reserves_stats

Bug 17931: Remove unused vars from reserves_stats

Change to Modern::Perl.
Remove null_to_zzempty and add a few blank lines between subs.
Remove unused vars: $podsp, $type, $daysel, $monthsel and $mime.
Removing trailing 1;

Test plan:
Run a few reports from reports/reserves_stats.pl

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Emma Smith <emma.nakamura.smith@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Wrongly pasted from reports/issues_stats.pl initially

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>