Bug 26579: Remove unused C4::Acquisition::DelOrder function

Bug 26579: Remove unused C4::Acquisition::DelOrder function

This patch removes an unused function, its tests, and adjusts other test
files that relied on it by replacing it with $order->cancel calls.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/Acquisition.t \
           t/db_dependent/Acquisition/GetBasketsInfosByBookseller.t \
           t/db_dependent/Acquisition/close_reopen_basket.t
=> SUCCESS: Tests pass!
2. Apply this patch
3. Repeat (1)
=> SUCCESS: Tests pass!
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>