Bug 14870: (followup) Remove stray C4::Dates from reports files

Bug 14870: (followup) Remove stray C4::Dates from reports files

This patch removes stray C4::Dates from following files:

reports/borrowers_stats.pl
To test: Verify that C4::Dates is not used in the file

reports/guided_reports.pl:
To test:
- Run a saved report with date input and with date range input, e.g.
- Verify that you are asked for date range or date and that the results
  are the same as without patch

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>