Bug 29717: Remove unecessary DT manipulations from tools/additional-contents.pl

Bug 29717: Remove unecessary DT manipulations from tools/additional-contents.pl

DBIC handles DateTime correctly, no need for this output_pref call.

Test plan:
Create a new content, set the dates, confirm they are set correctly
Modify the content, modify the dates, confirm they are stored correctly

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>