Bug 15769: Remove outdated C4::Koha::slashifyDate subroutine

Bug 15769: Remove outdated C4::Koha::slashifyDate subroutine

This subroutine does not make sense anymore (Koha::DateUtils should be
used) and is no longer used.
It can be removed safely.

Test plan:
  git grep slashifyDate
should not return any results

  prove t/Koha.t
and
  prove t/db_dependent/Koha.t
should return green

Signed-off-by: Hector Castro <hector.hecaxmmx@gmail.com>
slashifyDate removed, all test pass successfully

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com