Bug 14961: Remove C4::Dates from files reports/*_stats.pl

Bug 14961: Remove C4::Dates from files reports/*_stats.pl

Remove C4::Dates from following files:

- reports/acquisitions_stats.pl
- reports/borrowers_stats.pl
- reports/reserves_stats.pl
- reports/serials_stats.pl
- reports/issues_avg_stats.pl
- reports/issues_stats.pl

To test:
- Go to Home > Reports
- Verify that following reports work as before:
  - Other > Average loan time
  - Statistic wizards
    - Acquisitions
    - Patrons
    - Circulation
    - Serials
    - Holds

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>